List lines as ranges in font validator

[petzku]

For instance, the following:

warning: font 'Mercurius CT Std Medium' not found on line(s): 6539 6540 6541 6542 6543 6545 6546 6547 6548 6550 [...]

might print out something like

warning: font 'Mercurius CT Std Medium' not found on line(s): 6539-6543 6545-6548 6550-6554 [...]