We had one case where the merge() function returned successfully, but the MR
was not merged (needed a rebase actually):

https://gitlab.com/garz-fricke/yocto/infrastructure/ci-test/minimal-bar/-/jobs/1345574290

This seems to be a known issue:

https://gitlab.com/gitlab-org/gitlab-foss/-/issues/59577
https://gitlab.com/gitlab-org/release-tools/-/issues/277#note_154733089

It seems like in these rare cases, the error is reflected in the merge_error
field. Add evaluation of this to the error handling.