result in a cryptic error (and an abort):
  qemu: block-vvfat.c:97: array_get: Assertion `index < array->next' failed.
  Aborted

Turn this into an actually useful error message:
  Directory does not fit in FAT16 (capacity 504MB)
  qemu: could not open disk image fat:$DIR/


git-svn-id: svn://svn.savannah.nongnu.org/qemu/trunk@5665 c046a42c-6fe2-441c-8c8c-71466251a162