Fix message in the exception string

[Arun Mani]

Fix the error message substitution in the message string of the VirtualInterfacePlugException class.

When a VNIC attach fails, the exception handling block seems to be swallowing the actual error message that's coming from the platform.
Reason for this behavior is with the VirtualInterfacePlugException class definition which doesn't accept a formatting string and passing the same from the exception block shows only the generic message to the user and not the actual error message.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/608023

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/608023
Committed: https://git.openstack.org/cgit/openstack/nova-powervm/commit/?id=f7c7051ee98f5b61197b7e82cf5eb53d4ab7e68c
Submitter: Zuul
Branch: master

commit f7c7051ee98f5b61197b7e82cf5eb53d4ab7e68c
Author: arunmani <email address hidden>
Date: Thu Oct 4 12:37:08 2018 -0400

    Fix message handling with VirtualInterfacePlugException class usage

    When a VNIC attach fails, the actual error message thrown from the
    platform gets swallowed in the console showing only the generic
    message in the exception class. This is due to the formatting
    string passed to the exception class, though the
    VirtualInterfacePlugException class definition doesn't accept one.

    Change-Id: I18be7fc2284387d2f4d492ca68de6091f4bb8b66
    Closes-Bug: #1796072

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/nova-powervm 8.0.0.0rc1 release candidate.