Don't leave caller waiting for data if camera not enabled.

Author: mickp

microscope/devices.py

@@ -620,6 +620,8 @@ def grab_next_data(self, soft_trigger=True):
         :param soft_trigger: calls soft_trigger if True,
                                waits for hardware trigger if False.
         """
+        if not self.enabled:
+            raise Exception("Camera not enabled.")
         self._new_data_condition.acquire()
         # Push self onto client stack.
         self.set_client(self)