Print to stderr in on_error

Apparently the clever hack for logging in on_error was not so clever
after all.  If logging isn't configured, by the logging modules
definition of not configured, which is root logger not having an
Handlers attached, it will call logging.basicConfig().  Which messes up
setups that define handlers for other loggers than the root logger.

Going directly to the root logger rather than using the broken
convenience methods for logger is not an option either, as logger before
Python 3.2 does not have lastResort on the root logger, and prints an
error when invoked without any handlers.

Resolve by printing tracebacks to stderr by default in on_error.

discord/client.py

@@ -24,6 +24,8 @@ FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 DEALINGS IN THE SOFTWARE.
 """
 
+from __future__ import print_function
+
 from . import endpoints
 from .errors import InvalidEventName, InvalidDestination, GatewayNotFound
 from .user import User
@@ -34,6 +36,7 @@ from .message import Message
 from . import utils
 from .invite import Invite
 
+import traceback
 import requests
 import json, re, time, copy
 from collections import deque
@@ -482,7 +485,8 @@ class Client(object):
             raise InvalidDestination('Destination must be Channel, PrivateChannel, User, or str')
 
     def on_error(self, event_method, *args, **kwargs):
-        logging.exception('Ignoring exception in {}'.format(event_method))
+        print('Ignoring exception in {}'.format(event_method), file=sys.stderr)
+        traceback.print_exc()
 
     # Compatibility shim
     def __getattr__(self, name):