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: svn://svn.tartarus.org/sgt/putty@9745 cda61777-01e9-0310-a592-d414129be87e
Adjust comments around split_into_argv() to clarify that it's not
*Windows's* command-line splitting rules we're mimicking here; it's
VC7's, and they're not the same as VC10's.


git-svn-id: svn://svn.tartarus.org/sgt/putty@9748 cda61777-01e9-0310-a592-d414129be87e