xfs_scrub: fix read-verify pool error communication problems

xfs_scrub: fix read-verify pool error communication problems

Fix all the places in the read-verify pool functions either we fail to
check for runtime errors or fail to communicate them properly to
callers.  Then fix all the callers to report the error messages instead
of hiding them.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>