Propagate internal errors to the master node

This should help users diagnose internal errors in workers like
exceptions from hooks or in pytest itself.

src/xdist/workermanage.py

@@ -324,6 +324,8 @@ class WorkerController:
                 self.log("ignoring {}({})".format(eventname, kwargs))
             elif eventname == "workerready":
                 self.notify_inproc(eventname, node=self, **kwargs)
+            elif eventname == "internal_error":
+                self.notify_inproc(eventname, node=self, **kwargs)
             elif eventname == "workerfinished":
                 self._down = True
                 self.workeroutput = kwargs["workeroutput"]