MOAIFont crash fix, zl-gfx assert fix, and fix for silently failing to load the main script

Fixes  moai.exe crashes when attempting to use an unloaded font ( #978 )
Fixes a debug-only assert in zl-gfx due to improper use of an std::string
Makes the moai.exe host log an error when the main script couldn't be loaded instead of silently failing