engine : Fix ResourceFailure exception issues

Fix some issues spotted in https://review.openstack.org/#/c/29543
These errors were spotted after the patch was merged, the
ResourceFailure exception takes an Exception not string argument

Change-Id: I72045ecf147c66a0a1cf1ef5df7fb3be8100384d
Make volume attachment a co-routine

Change-Id: Iad1ac0578470e4c2f997257560d62f8cb2d7ee35
Merge "Make volume attachment a co-routine"
Merge "engine : Fix ResourceFailure exception issues"