Loading spinner not stop even on errors in updating Milestone on Issue

What does this MR do?

Adds error handling on Milestone update on Issue detail by stopping the loading spinner, which was introduced in !3175 (merged).

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

There is no error handling on Milestone update on Issue detail.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [n/a] Documentation created/updated
• [n/a] API support added
• [n/a] Tests added for this feature/bug
• Review
  • [n/a] Has been reviewed by UX
  • Has been reviewed by Frontend
  • [n/a] Has been reviewed by Backend
  • [n/a] Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• [n/a] Internationalization required/considered
• [n/a] End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #43837 (closed)