Avoid UnboundLocalError for exc

Previously exc was set only when an exception occurs in the inner
try (the one contains call to self.poll), so an error in the other
part causes an UnboundLocalError.

Closes #178

websockify/websocket.py

@@ -1047,6 +1047,7 @@ def start_server(self):
                         self.msg("In exit")
                         break
                     except Exception:
+                        exc = sys.exc_info()[1]
                         self.msg("handler exception: %s", str(exc))
                         self.vmsg("exception", exc_info=True)