File loading problems

[tryashtar]

So, it seems like there are two different "empty" region files that are failing to load:

• Actually empty, zero-length files
• Files with a full header, but all 8196 bytes are zeros. And then some trailing data sometimes

Both of these should load as empty regions. But... isn't it a bad idea to treat every empty file as a region file? Yes, that's why we should do this:

• Guess the file format from the file extension: .nbt is probably binary NBT... .mca is probably a region file
• Try to load it that way, with some special logic. Empty files just get new'd up. Region files bypass the one-chunk check
• If that fails (or unrecognized extension), try all other ways of loading the file, without the special logic

Also we need to fail faster at file type verification; exceptions are cringe. You already did both of these things in Quick Music so just do that again