Add a page that shows all the bots that fail to load results in the flakiness dashboard.

I'd like to drive this number down to zero. Having a list of issues to
drive down is the starting place.

Mostly this just requires exposing a few more bits from builders and
loader. Otherwise, it's just going through the motions of loading
each result for testtype/group combination and seeing which ones
fail.

NOTRY=true

Review URL: https://codereview.chromium.org/293393006

git-svn-id: svn://svn.chromium.org/blink/trunk@174991 bbb929c8-8fbe-4397-9dbb-9b2b20218538