Add sparsebundle_fatal_error function for unifying fatal errors

The function is marked with the printf archetype, so that the compiler
will not complain that we're passing non-string-literals to vfprintf.
Handle path verification of bundle and mount point in sparsebundle_opt_proc
Clarify difference between bundle and exposed dmg path in logging

It's not the bundle that's opened by fuse, that's just our internal
pointer back to where the data lives. As far as fuse is concerned
the only file we have is sparsebundle.dmg, and that's the file that's
opened and released.