Promote "Unknown instruction format" warning to an error

[neobrain]

e.g. a like like ; invalid-comment will trigger the warning source/vshader.vsh:15: warning: Unknown instruction format, treating like EOF: " and then abort parsing. Instead, this should be considered an error.

[machinamentum]

This change seems simple (std::cerr output -> throw), but could you elaborate on the behavior caused by Windows EOF? I don't have a Windows machine, so I can't extensively explore this.