Reorganise setup_fonts_ucs so that in case of error it does nothing

Reorganise setup_fonts_ucs so that in case of error it does nothing
and returns its error message as a string, instead of actually
printing it on standard error and exiting. Now we can preserve the
previous error behaviour when we get a nonexistent font name at
startup time, but no longer rudely terminate in mid-session if the
user configures a bogus font name in Change Settings.

git-svn-id: http://svn.tartarus.org/sgt/putty@9745 cda61777-01e9-0310-a592-d414129be87e