mediawiki::monitoring::errors: report to statsd

mwerrors.py is a script that listens to the UDP log stream coming from the app
servers, picking out individual error subtypes, and incrementing the
appropriate metric. It is the script that generates the MediaWiki error count
graph that is mentioned in the channel topic of #wikimedia-operations.
This patch changes it from being a gmond module that reports to Ganglia to a
standalone script that reports to StatsD. (We didn't have StatsD back when I
wrote it.)

Change-Id: I4cafb0d347d9af1966a90e9baa788703bc9c2f7b
RT: 7699