Merge request veto error wrongly shows merge conflict resolution instructions

Summary

When an attempt to merge a pull request is vetoed (after the UI checks the merge using dry-run), the error message shown is incorrect.

Steps to Reproduce

1. Open a mergeable pull request in the browser
2. In another tab, enable a plugin which always vetoes merges
3. Merge the pull request
4. Click 'view details' on the error message

Expected Results

I see the veto message from the plugin

Actual Results

I'm shown instructions on how to resolve a merge conflict