frontend: don't traceback on invalid cancel requests
Flash in web-UI for ConflictingRequest raised by cancel request, but
also avoid another cli failure:
$ copr cancel 51501
Something went wrong:
Error: Request wasn't successful, there is probably a bug in the API code.
Newly we write:
$ copr cancel 51501
Something went wrong:
Error: Cannot cancel build 51501
Fixes: #1325