Resolve "Improve error message when attempting to reopen MR and there's a new open MR for the same branch"

What does this MR do?

Fixes #22548 (closed): When attempting to reopen MR and there's a new open MR for the same branch, we show an error message in flash instead of silently failing.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered

What are the relevant issue numbers?

Closes #22548 (closed)