Fixes: https://github.com/go-gitea/gitea/issues/29956. This error
exposed a existing bug in the code, it was just never noticed because
the jQuery expression evaluated without error before while the new one
doesn't.
Also improves error message styles:
Before:
<img width="1338" alt="Screenshot 2024-03-21 at 09 16 07"
src="1cc1ef89-ad94-491e-bbca-75387f7547a0">
After:
<img width="1335" alt="Screenshot 2024-03-21 at 09 15 44"
src="312efc79-5353-4e2a-a703-1bccd3c01736">
(cherry picked from commit 1a4f693f9f9723c181b747cb6d658aa37118005a)