Non-existing test pages, Histogram of failures, and Histogram of skips

[jjchen]

• design a back-end interface for retrieval of distribution of semantic & syntactic errors & failed fetches and implement it (query on test_by_score)
• hook up the front-end to display these lists.

Current interfaces:
http://parsoid.wmflabs.org:8001/failedFetches
http://parsoid.wmflabs.org:8001/failsDistr
http://parsoid.wmflabs.org:8001/skipsDistr

[jjchen]

failsDistr and skipsDistr basically finished: jjchen@37327aa, just need to refactor.