PATENT CLAIMS 1. A system for rejection of tablets produced in a rotary tablet press, comprising an ejection nozzle for direction of a gas jet at a tablet to be rejected, so that the tablet is redirected and thereby sorted out from the remaining tablets leaving the tablet press during production, the ejection nozzle being supplied with compressed gas through a supply line having an inlet end for connection with a source of compressed gas and an outlet end communicating with the ejection nozzle, the supply line being provided with a control valve controlled by a control unit in order to open the supply line when a tablet to be sorted out has been detected by the control unit, and the supply line being provided with a sensor adapted to detect a variable indicative of a gas flow through the control valve in order for the control unit to generate an error signal in the case of malfunction of the control valve, characterized in that the sensor is arranged in the supply line between the control valve and the inlet end of the supply line. 2. A system for rejection of tablets according to claim 1, wherein the control valve is arranged next to the ejection nozzle. 3. A system for rejection of tablets according to claim 1 or 2, wherein the system comprises an ejection finger adapted to guide tablets from a die table of the tablet press into a first channel of a tablet chute, and wherein the ejection nozzle is arranged in the ejection finger in order to direct tablets into a second channel of the tablet chute. 4. A system for rejection of tablets according to claim 3, wherein the ejection finger has a fixed end adapted to be mounted beside the die table and a free end to be located above the die table, and wherein the control valve is located at the free end of the ejection fin ger. 5. A system for rejection of tablets according to claim 3 or 4, wherein the control valve is integrated in the ejection finger. 6. A system for rejection of tablets according to claim 4 or 5, wherein the sensor is arranged at the fixed end of the ejection finger. 7. A system for rejection of tablets according to any one of the claims 4 to 6, wherein the fixed end of the ejection finger is mounted on the top of a pole mounted on a dividing wall of a tablet press, and wherein the sensor is arranged below the dividing wall and under the pole. 8. A system for rejection of tablets according to claim 7, wherein the supply line is integrated into the pole and the ejection finger. 9. A system for rejection of tablets according to any one of the preceding claims, wherein the sensor is a pressure sensor, and wherein the control unit is adapted to generate an error signal, if a predetermined drop of the pressure measured by the pressure sensor is not detected by the control unit, when the control valve is opened. 10. A system for rejection of tablets according to claim 9, wherein the predetermined drop of pressure is at least 2 per cent, and preferably at least 3 per cent. 11. A system for rejection of tablets according to claim 9 or 10, wherein the control unit is adapted to monitor the pressure measured by the pressure sensor, when the control valve is closed, and to generate an error signal in case that the pressure goes under a minimum pressure. 12. A system for rejection of tablets according to any one of the preceding claims, wherein the sensor is a flowmeter, such as a differential pressure flowmeter, an ultrasonic flowmeter, a target flowmeter, a coriolis mass flowmeter or any kind of suitable flowmeter. 13. A rotary tablet press comprising a system for rejection of tablets according to any one of the preceding claims. 14. A method of rejection of tablets produced in a rotary tablet press, whereby a gas jet is directed at a tablet to be rejected, so that the tablet is redirected and thereby sorted out from the remaining tablets leaving the tablet press during production, whereby the compressed gas is supplied to the ejection nozzle through a supply line from a source of compressed gas, whereby a control valve arranged in the supply line opens for the supply of gas when it receives a control signal from a control unit, the control unit sending the control signal to the control valve when it has detected a tablet to be sorted out, whereby a sensor arranged in the supply line detects a variable indicative of a gas flow through the control valve, and whereby the system generates an error signal in the case of malfunction of the control valve, c h a r a c -t e r i 2 e d by that the variable is detected by the sensor in the supply line from the source of compressed gas to the control valve. 15. A method of rejection of tablets according to claim 14, whereby the control valve opens for the gas flow next to the ejection nozzle. 16. A method of rejection of tablets according to claim 14 or 15, whereby successful tablets are guided by means of an ejection finger from a die table of the tablet press into a first channel of a tablet chute, and whereby defective tablets are directed into a second channel of the tablet chute by means of a gas jet ejected through an ejection nozzle arranged in the ejection finger. 17. A method of rejection of tablets according to any one of the claims 14 to 16, whereby the control valve is opened inside the ejection finger. 18. A method of rejection of tablets according to any one of the claims 14 to 17, whereby the variable detected by the sensor is a pressure signal, whereby the control unit generates an error signal, if a predetermined drop of the pressure measured by the pressure sensor is not detected by the control unit, when the control valve is opened. 19. A method of rejection of tablets according to claim 18, whereby the predetermined drop of pressure is at least 2 per cent, and preferably at least 3 per cent. 20. A method of rejection of tablets according to any one of the claims 14 to 19, the pressure measured by the pressure sensor is monitored by the control unit, when the control valve is closed, and an error signal is generated in case that the pressure goes under a minimum pressure. 21. A method of rejection of tablets according to any one of the claims 14 to 17, whereby the variable detected by the sensor is flow signal, and whereby the flow signal is measured by means of a differential pressure flowmeter, an ultrasonic flowmeter, a target flowmeter, a coriolis mass flowmeter or any kind of suitable flowmeter. 22. A method of tabletting comprising a method of rejection of tablets according to any one of the claims 14 to 21.