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.

docs/logging.rst

@@ -4,11 +4,10 @@
 Setting Up Logging
 ===================
 
-*discord.py* logs errors, exceptions, and debug information via the
-`logging`_ python module. It is strongly recommended that the logging
-module is configured, as no errors or warnings will be output at all if
-it is not set up. Configuration of the ``logging`` module can be as
-simple as::
+*discord.py* logs errors and debug information via the `logging`_ python
+module. It is strongly recommended that the logging module is
+configured, as no errors or warnings will be output if it is not set up.
+Configuration of the ``logging`` module can be as simple as::
 
     import logging