blkrawverify: warn and return error if no traces are found

blkrawverify: warn and return error if no traces are found

blkrawverify is prints no errors and returns success if the
requested tracefiles aren't found:

# blkrawverify foobar
Verifying foobar
# echo $?
0

With this change it's a bit more informative:

# ./blkrawverify foobar
Verifying foobar
No tracefiles found for foobar
# echo $?
1

Resolves Red Hat Bugzilla #499581

Reported-by: Milos Malik <mmalik@redhat.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>