Report proper error message in PLUMgrid Plugin

Change-Id: Ifc1bb55f6b025bba77cf9858ed392dbf170075a7
Closes-Bug: #1268460
Signed-off-by: Fawad Khaliq <fawad@plumgrid.com>
(cherry picked from commit 5fae0fae2164f4fab64e3eff01c9c644d861946a)
Merge "Report proper error message in PLUMgrid Plugin" into stable/havana
Merge branch 'stable/havana' of https://github.com/openstack/quantum into havana/stable