a0063f9df0 slightly improved the error
message on non-PR ci failure (e.g. a community PR makes enterprise
break) by adding the failed commit, but that's still not exactly clear,
even for technical users (plus it requires having access to all the
repos which is not the case for everyone).
This commit further improves the situation by storing the target_url
and description fields of the commit statuses, and printing out the
target_url on failure if it's present.
That way the PR comment denoting build failure should now have a link to
the relevant failed build on runbot, as that's the target_url it
provides.
The change is nontrivial as it tries to be compatible with both old and
new statuses storage format, such that there is no migration to perform.