A bug in python-gitlab > 2.7.0 [1] brought this up: In the past we
relied on the merge request's setting, whether the source branch should
be deleted or not after a merge. Since we're using the API's merge
function in a fully automated task, however, the branch should always be
deleted. Add the according argument explicitly.

[1]: https://github.com/python-gitlab/python-gitlab/issues/1750

* Combine two return statements into one.
* Switch if statement order to remove else branch.

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.