[WIP] [FIXED JENKINS-11172] Don't throw exception on /signup when not possible

Nothing links there, just don't show the page that tells users to report a bug.
I see, I did not realize `HudsonPrivateSecurityRealm/signup.jelly` is written to assume that signup is enabled, and does not check otherwise. There are ways to differentiate between a realm that could allow signup but currently does not, vs. one which inherently does not, but for purposes of this PR I guess that is overkill.