Resolving a merge conflict with restrictions on commit names shows generic error

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Summary: When a repository have a restriction on commit names, resolving a merge conflict without respecting this restriction leads to a generic "An error occurred" error.

How to reproduce:

1. Define a commit name restriction
2. Create a merge request
3. Create a merge conflict on the target branch
4. Try to resolve the conflict via the MR page, without respecting the restriction

What happens: A generic "An error occurred" error is displayed

What should happen: A more detailed error like "The commit does not match the regex" (like displayed when pushing) should be displayed

Instance details: Self-hosted v18.3.1-ee