Fix wrong exception message when deploy failed
Fix wrong exception message when deploy failed

Currently, if conductor gets an exception while trying to deploy the
node, we will get an exception message like:

Asynchronous exception for node $UUID: Node failed to get image for
deploy. Exception: Failed to continue agent deployment.

It's not right because the conductor may successfully fetch the image
and fail in other reasons, like fail to connect to neutron.

Change-Id: Ide4bb749365709a3ec321b74a58abdc0006e34b8

openstack/ironic,openstack/ironic,NaohiroTamura/ironic,pshchelo/ironic,SauloAislan/ironic,SauloAislan/ironic,NaohiroTamura/ironic,pshchelo/ironic