Error on opening unsupported file

[jacksongoode]

Add notice that opening a particular file type is not supported instead of opening nothing with no dialog.

[Introscopia]

Hey! Sorry I never replied to these, I'm awful with github notifications.
Although I like this suggestion in principle, I won't do it because on this project the main principle is minimalism. Having any kind of dialog means loading a font and a UI library.
Although.. it just occurred to me I can write the message to the window title! Yeah, I think I'll do that..

[jsoulier]

There's also SDL_ShowSimpleMessageBox to put something onscreen.