[POWERPC] boot: More verbose gunzip error message

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: Milton Miller <miltonm@bga.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>