Better import screen error messages

[cxong]

When the import screen command fails, the error message doesn't tell why, so if someone is unfamiliar with this function, it takes trial and error to figure out why.

I suggest some of the following implemented:

• Better documentation on wiki for the exact requirements of import screen
• In the code, implement specific error messages like:
  • Image not png (or supported format)
  • Image not loaded, plus error message from decoder if available
  • Image not the right dimensions (or allow TIC-80 to handle different dimensions, via cropping, scaling or letterboxing)

[Skeptim]

Thanks
Here are my corrections to the wiki from your comments. Tell me if something is still missing.

I tested to import .gif and .jpg for cover image, sprites or tiles it did not work. I guess only png works but if someone knows of an other format that can be used, please tell me.