Use validation error messages for branch rules

The following discussion from !175032 (merged) should be addressed:

• @vyaklushin started a discussion:

  issue (non-blocking): After I set a squash option for a protected branch and try to change the name of the branch to a wildcard, then I see a generic error. We're probably missing an error handling on frontend side for this case.

  

  

Requirements

• Expose the error message returning from backend

Implementation Plan

• In app/assets/javascripts/projects/settings/branch_rules/components/view/index.vue, we only createAlert with a generic message when editBranchRule mutation returns any errors.
• Update the error message returning to match the suggestions in #517472 (comment 2822619743)