Propagate error on FF pre-receive failure

What does this MR do?

Ensure pre-receive hook errors are propagated to the GUI when encountered on fast-forward merges.

The Error

The response in Gitlab::Gitaly::GitalyClient::OperationService#user_ff_branch returns <Gitaly::UserFFBranchResponse: branch_update: nil, pre_receive_error: "pre-receive hook error message\n">. This isn't raised further, nor is it passed into the final call in this method to Gitlab::Git::OperationService::BranchUpdate#from_gitaly. Instead only response.branch_update is passed in, which is nil.

Related #25228 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team