Fix API integrity on rename

[jesperhodge]

• When renaming a tag to a name that's duplicate, I just want to get a 4xx error, not an HTML integrity error page
• The API generally should hide internal errors from the end user and just send 500 if something unexpected / unhandled happens

[jmakowski1123]

As a non-technical tester, is this the error you expect to see?

[jesperhodge]

Hi @jmakowski1123 ! Indeed, expected behavior is that you should see an appropriate error message explaining that this tag is a duplicate (or what other reason there is that it's invalid).

Since this ticket was done on the backend repository, but the fix for the message has to be done on the frontend / user side, I have not included it here.

However, I have found this problem as well; and my fixes to the error messages in #272 have addressed it as well. I already fixed it in my implementation, and when that is merged, the error message problem you found should be resolved.

I have asked Tammie to include it in the ACs for 272 so that we know to test for it.