FavIcon now quietly handles empty icon URLs, and is more graceful when lacking fileinfo extension.

Also, install.php will now warn if PHP is missing the fileinfo extension.  If it is not available, FavIcon will now allow some well-known extensions through blindly, even if they are served with incorrect content-types, rather than rejecting them.
Fixed an issue when rendering errors in fof_update_feed().

Was checking isset() instead of for false.  False variables are still set, it turns out.