Report MediaWiki exceptions & fatals to Ganglia

This patch re-introduces the Python Ganglia module for MediaWiki exceptions and
fatals which I removed in If035a0ef2 so I could clean up the EventLogging
module. I've modified it to consume the UDP error stream directly rather than
via ZeroMQ forwarding since it was adding an unnecessary layer of complexity to
a piece of code that should be as straightforward as possible. Other than the
slight modification to consume a UDP stream rather than a ZeroMQ stream, the
code is substantially the same as it was when Tim reviewed it.

Change-Id: I874ec534062c2086f2c1ac00bdb9cb4ffbeaf846