Use the cherry-pick error code returned by API
When the GitLab API fails to cherry-pick, it now includes an
error_code
field in the JSON response.
I'm hoping that the gitlab gem will just make this field available as a method on the Error object, as it does for arbitrary fields on other resources.
If it doesn't, we fall back to the :failure
code.
This is a bit of research to see what we can do for https://gitlab.com/gitlab-org/release-tools/issues/337.