Do not assume a failure message is specified

Not everyone needs a custom failure message, so
if nothing is specified create an empty node.

Change-Id: I82f69e28574c01cbc6de649f968482352f42b8d8
Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Reviewed-on: https://review.openstack.org/36229
Reviewed-by: Arnaud Fabre <fabre.arnaud@gmail.com>
Reviewed-by: Khai Do <zaro0508@gmail.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins