The latest version of fontspec now returns proper error messages when a font cannot be found:

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
./small.tex:6:
! fontspec error: "font-not-found"
!
! The font "Ggeorgia" cannot be found.
!
! See the fontspec documentation for further information.
! For immediate help type H <return>.
!...............................................

l.6 \fontspec{Ggeorgia}

? H
|'''''''''''''''''''''''''''''''''''''''''''''''
| A font might not be found for many reasons.
| Check the spelling, where the font is installed etc. etc.
|
| When in doubt, ask someone for help!
|...............................................

The wording might not be optimal, but having something to explain what’s going on here has been missing for a long time!

The ASCII-art ‘design’ of these error messages comes from work we’ve been doing in expl3 trying to make the console output a little more attractive. It’s not an easy job working around TeX’s idiosyncrasies here, but I’m quite happy with the final result.