Add assert to ensure a file exists.
Add assert to ensure a file exists.

Took me a while to track down that I had a typo in my file name, because the
exception thrown was from NSRegularExpression. So add an `NSAssert()` to save
other future developers from themselves.

Joulebug/nui,Jake000/nui,mutualmobile/nui,TribeMedia/nui,jianwoo/nui,tutoru/nui,timbodeit/nui,AnyPresence/nui,tutoru/nui,Jake000/nui,foxsofter/nui,colemarkham/nui,peantunes/nui,tombenner/nui,antobito11/nui,foxsofter/nui,jianwoo/nui,AaronYi/nui,Jake000/nui,colemarkham/nui,TribeMedia/nui,foxsofter/nui,AaronYi/nui,Neosperience/nui,ivanyang1984/nui,tutoru/nui,Suninus/nui,KaterinaPetrova/nui,Suninus/nui,Stunner/nui,jianwoo/nui,ivanyang1984/nui,AaronYi/nui,ivanyang1984/nui,colemarkham/nui,Suninus/nui,Neosperience/nui,odigeoteam/nui,TribeMedia/nui,Neosperience/nui,jianwoo/nui