Improve error handling in PXE _continue_deploy

Related to bug 1184470, there was a concern that the PXE driver
may not be adequately handling errors and informing users when failures
occur mid-deploy.

This patch refactors the _continue_deploy() method to handle both errors
POSTed from the ramdisk and errors that originate within deploy_utils.

It also fixes an inconsistency in the final provisioning_state:
ConductorManager.do_node_deploy() will set provisioning_state = ACTIVE,
however the PXE driver was leaving nodes with state = DEPLOYDONE.

Change-Id: I29cbff87cbaf85d95687ae094720f8b99f33b65f
Related-bug: 1184470
Merge "Improve error handling in PXE _continue_deploy"