Add alerting for MediaWiki exceptions and fatals
Watch the rate per minute of MediaWiki exceptions and fatals. Warn if it
exceeds 25; panic if it exceeds 50. I hope we can gradually make these numbers
lower, but for now we need to make sure we're not too noisy or we'll
inadvertantly train people to ignore the alert.
Bug: T140942
Change-Id: I638d270e52a559a5b6bc0f68788172869ca2d888