Make gbagfx give some better error messages

[LOuroboros]

I noticed that while gbagfx works with .png image files, if for whatever reason you have one of them named as (File).PNG, gbagfx won't work.
That's why I suggest to make it give better error messages that can make troubleshooting easier for the Users.

In this case, something like "Error: Your image file is not valid. Please make sure that its file extension is written in lowercase letters." would do, for example.

[aaaaaa123456789]

That's just one of many possible ways of writing the extension incorrectly. A much better error would point out that the extension wasn't recognized, and show the valid ones.

[LOuroboros]

It was just an example, but I absolutely agree.

[RevoSucks]

do we really need this?

[LOuroboros]

Unless you want people having to figure out on their own the reason why their sprite is not being converted with gbagfx (in case anyone randomly happens to come across this problem just like me), I'd say it would be a good thing to have, @ProjectRevoTPP.