Change the error message in gunzip_exactly to be more verbose.

Besides the identifier being unrelated to the current function name,
the user had no indication if the corruption was near the beginning
or the end.
Signed-off-by: NMilton Miller <miltonm@bga.com>
Acked-by: NDavid Gibson <david@gibson.dropbear.id.au>
Signed-off-by: NPaul Mackerras <paulus@samba.org>