Catch exceptions in list_issues reactive cache calculation

What does this MR do?

Ensure that exceptions are caught when calculate_reactive_cache is called and return the error to the FE.

When a Sentry API returns a non-200 status code, Sentry::Client raises Sentry::Client::Error. This MR catches the exception in the ProjectErrorTrackingSetting model so that an appropriate error message can be sent to the FE.

After this fix, the following error message will appear on the Operations > Error Tracking page if the Sentry API fails.

What are the relevant issue numbers?

#56871 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Documentation reviewed by technical writer or follow-up review issue created
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer

Closes #56871 (closed)