Better handle exceptions when notifying sessions

Based on #1022

Author: Neil Booth

electrumx/server/session.py

@@ -956,6 +956,15 @@ def unsubscribe_hashX(self, hashX):
         return self.hashX_subs.pop(hashX, None)
 
     async def notify(self, touched, height_changed):
+        '''Wrap _notify_inner; websockets raises exceptions for unclear reasons.'''
+        try:
+            await self._notify_inner(touched, height_changed)
+        except CancelledError:
+            raise
+        except Exception:
+            self.logger.exception('unexpected exception notifying client')
+
+    async def _notify_inner(self, touched, height_changed):
         '''Notify the client about changes to touched addresses (from mempool
         updates or new blocks) and height.
         '''