Improve error handling in validate_vendor_action

validate_vendor_action method of conductor may be used for validate
callback data from deploy/maintenance ramdisk, error message not
saved in Ironic for this case.
This patch adds saving for error message to node.last_error.

Change-Id: Ic01a6f9a95c7e09e7ce7925dad9a7fd228de65bd
Merge "Improve error handling in validate_vendor_action"