Clarify error messages about bad inodes.

When something was found wrong with an inode the error message
was always "UNKNOWN FILE TYPE". This error is now used only when
the file type field is wrong. Other errors have their own messages:
"BAD FILE SIZE", "NEGATIVE FILE SIZE", "BAD SPECIAL-FILE RDEV",
"INVALID DIRECT BLOCK", and "INVALID INDIRECT BLOCK".

More complete information about the inode is also provided.

Sponsored by: The FreeBSD Foundation