Better name for this function.
Better error message if file is malformed.

The file that caused this wasn't malformed, but the errors that issue
from malformed data are useless to debug with. Introducing a new class
that contains an informative message and the stream position, so that
you can see where it was when parsing went south.
Check for TAG_Lists with invalid types.

Note: the TAG_End thing is temporary, as we do know these to exist in
the while in current (1.7.10) versions of Minecraft.