Gerrit: CI should post correct failure comments into Gerrit

When changes are in state "INTEGRATING", and the CI system posts a failure and moves them back to unstaged, a comment should be added to all the failed changes.

The comment should have at least the following:

• approximately the same content as the current CI system failure reports (e.g. the relevant build messages extracted from Pulse, the autotest regressions and details, and any "hints")
• a link to the raw build log(s)

Note that the latter depends on QTQAINFRA-139.