Raise an error if creating the MergeRequestDiff fails

What does this MR do?

We're seeing some backtraces that imply MRs are being created without attendant MR diffs. I suspect the actual problem is being masked by the fact that we don't raise an error if creating the MR diff fails on MR creation.

This MR makes it into a failing operation, so we should see why creating an MR diff fails - we can then address those root causes as they appear in logs.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance 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

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

Related to #62690 (moved)