Make HPIB raise a RuntimeError no 'no response'

hpib.py

@@ -44,7 +44,10 @@ class GPIBProtocol(serial.threaded.LineReader):
 
         Returns:
              If the command contains '?', the value returned by
-             the HPIB device is returned, else None.
+             the HPIB device is returned.
+
+        Raises:
+             RuntimeError: if no response was received to a query.
         """
         with self.lock:
             if address:
@@ -56,7 +59,7 @@ class GPIBProtocol(serial.threaded.LineReader):
                 try:
                     line = self.responses.get(timeout=timeout)
                 except queue.Empty:
-                    return None
+                    raise RuntimeError("HPIB device at address did not respond to query:", command)
                 return line
             else:
                 pass