qemu-img: Print error if check failed

Currently, if bdrv_check() fails either by returning -errno or having
check_errors set, qemu-img check just exits with 1 after having told the
user that there were no errors on the image. This is bad.

Instead of printing the check result if there were internal errors which
were so bad that bdrv_check() could not even complete with 0 as a return
value, qemu-img check should inform the user about the error.

Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Markus Armbruster <armbru@redhat.com>