🐛 FIX: retrieve future exception on_killed (#206)

The exception set on the future should be retrieved,
otherwise it will be caught by the loop's exception handler.

plumpy/processes.py

@@ -823,6 +823,7 @@ def on_kill(self, msg: Optional[str]) -> None:
     def on_killed(self) -> None:
         """Entered the KILLED state."""
         self._killing = None
+        self.future().exception()  # exception must be retrieved
         self._fire_event(ProcessListener.on_process_killed, self.killed_msg())
 
     def on_terminated(self) -> None: