Log to root logger by default in on_error

Change the default implementation of on_error to log to the root logger
instead of discord.client and clarify that the exception is being
ignored.  This ensures that a message will be output to standard error
in case the logging module has not been configured.

Also removes the argument printing for the default on_error, this is due
to them often being too long, that they could cause another exception to
be thrown, and because it sometimes causes sensitive information to be
output such as Discord tokens and session ids.  It was also possible for
the length to get in the megabyte range with exceptions thrown by
on_socket_raw_receive in READY events.

discord/client.py

@@ -478,8 +478,7 @@ class Client(object):
             raise InvalidDestination('Destination must be Channel, PrivateChannel, User, or str')
 
     def on_error(self, event_method, *args, **kwargs):
-        msg = 'Caught exception in {} with args (*{}, **{})'
-        log.exception(msg.format(event_method, args, kwargs))
+        logging.exception('Ignoring exception in {}'.format(event_method))
 
     # Compatibility shim
     def __getattr__(self, name):