Errors from the callees must be captured and propagated to our caller,
ensure this for both find_extent() and bdrv_getlength().

Reported-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Fam Zheng <famz@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>