Change font to default font and invalidate the font cache if the current font doesn't exist anymore

[forkiesassds]

Related with #928.

[UnknownShadow200]

At least 1 of the 11 fallback system fonts should be missing on any system out there, hence at least one call will be made to SysFonts_Update() to retrieve updated list of fonts from disc already - so theoretically the outdated fonts issue should automatically be resolved next time game is started

(was unsuccessful in reproducing issue on Windows, because even when I manually changed the paths of all system fonts in fontscache.txt, they were automatically corrected next time the game was started)