Buildbot log fetching doesn't work for --always-succeed jobs

[jdm]

https://github.com/servo/highfive/blob/master/handlers/homu_status/__init__.py#L27-L37 looks for a buildbot job that fails, then finds the stdio log for the failed job and prints that. For jobs like the bluetooth tests that fail if a test fails, that works perfectly fine. For jobs like the rest of the WPT tests, where the actual test run job passes and a subsequent log filtering job fails (and doesn't print anything to stdio) this does not work.

[jdm]

A quick fix would be to look for any log with -errorsummary.log in the name as well; sadly, that output is less generally helpful than the actual WPT job stdio.

[jdm]

Basic support added in #207. It would be nice to get the original "succeeding" job's output instead.