Change behaviour of on_error

Change how the old style on_error event is called to match the new style
on_error event.  Both are now called in case an exception is raised in
an user defined event handler, and will by default print the arguments
of the event tha raised the exception and the traceback for the
exception.  In addition, overridding the on_error handler supresses this
behaviour.

discord/client.py

@@ -360,7 +360,7 @@ class Client(object):
         self.events = {
             'on_ready': _null_event,
             'on_disconnect': _null_event,
-            'on_error': _null_event,
+            'on_error': self.on_error,
             'on_response': _null_event,
             'on_message': _null_event,
             'on_message_delete': _null_event,
@@ -457,8 +457,7 @@ class Client(object):
             log.info('attempting to invoke event {}'.format(event_name))
             self.events[event_name](*args, **kwargs)
         except Exception as e:
-            log.error('an error ({}) occurred in event {} so on_error is invoked instead'.format(type(e).__name__, event_name))
-            self.events['on_error'](event_name, *sys.exc_info())
+            self.events['on_error'](event_name, *args, **kwargs)
 
     def handle_socket_update(self, event, data):
         method = '_'.join(('handle', event.lower()))