Raise error if patch preparation branch fails to be created

What does this MR do?

Raise an error if the preparation branch for a patch release fails to be created.

We sometimes get 400 errors even though an internal server error occurred.

Job where the 400 error occurred: https://ops.gitlab.net/gitlab-org/release/tools/-/jobs/6384122

Error when creating preparation branch:

2022-02-23 10:49:29.293693 W ReleaseTools::PreparationMergeRequest -- Failed to create preparation branch -- {:name=>"14-8-stable-ee-patch-1", :target=>"14-8-stable-ee", :project=>gitlab-org/gitlab, :error_code=>400, :error_message=>"Server responded with code 400, message: 500 Internal Server Error. Request URI: https://gitlab.com/api/v4/projects/gitlab-org%2Fgitlab/repository/branches"}

Author Check-list

• Has documentation been updated?