This is implement in an extremely hacky way due to poor DBIx feature
support. Ideally, what we'd need is a way to tell DBIx to ignore the
errormsg column unless explicitly requested, and to automatically add a
computed 'errormsg IS NULL' column in others. Since it does not support
that, this commit instead hacks some support via method overrides while
taking care to not break anything obvious.
Depends on https://github.com/nix-community/nix-eval-jobs/pull/349 & #1421.
Almost equivalent to #1425, but with a small change: when having e.g. an
aggregate job with a glob that matches nothing, the jobset evaluation is
failed now. This was the intended behavior before (hydra-eval-jobset
fails hard if an aggregate is broken), the code-path was never reached
however since the aggregate was never marked as broken in this case
before.
