Exit stalled requests on connection error.

If the SSH connection stalled or just existed during a restore, the whole process would stall as it waits for requests which will never finish.

This MR makes the connection propagate the `ConnectionQuit` exception to stalled requests when the connection is cut in the middle of the process, so they will terminate with an exception and raise it, exiting the whole program instead of making it stall

/reviewed-on !6

rdiff_backup/connection.py

@@ -341,7 +341,10 @@ class PipeConnection(LowLevelPipeConnection):
 		while True:
 			try:
 				req_num, obj = self._get()
-			except ConnectionQuit:
+			except ConnectionQuit as e:
+				# Terminate all pending requests
+				for req in self.requests.values():
+					req.set(e)
 				self._close()
 				break