Ratcheting hides duplicate errors (sometimes)

When working on Twisted ticket https://tm.tl/#6803 I noticed that some pydoctor errors were missing. That is, I saw one pydoctor error:

twisted.conch.ssh.keys.Key.toString: invalid ref to NoneType

... but the same module has many more identical refs to NoneType, so I would expect multiple such failures.

tomprince suggested these may be buildbot configuration failures, not pydoctor ones:

14:58 < tomprince> Also, the new error reporting tends to collapse identical errors.
15:18 < tomprince> lvh: I mean the stuff that does racheting in buildbot.