pid_calibrate: Avoid internal error on a shutdown

If a shutdown occurs during a PID_CALIBRATE command (eg, due to a
heater problem) then exit cleanly instead of throwing an internal
error.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/pid_calibrate.py

@@ -34,6 +34,8 @@ class PIDCalibrate:
         heater.set_control(old_control)
         if write_file:
             calibrate.write_file('/tmp/heattest.txt')
+        if calibrate.check_busy(0., 0., 0.):
+            raise gcmd.error("pid_calibrate interrupted")
         # Log and report results
         Kp, Ki, Kd = calibrate.calc_final_pid()
         logging.info("Autotune: final: Kp=%f Ki=%f Kd=%f", Kp, Ki, Kd)