Send proper exception info as expected by the neutron client

The python neutron client for the V2 API expects the neutron API
to send information back such as the type and detail of the exception
in the body of the message

Change-Id: I9486d757258c4be72799c41102babe1f7923121c
Closes-bug: #1226400
Allowed Address Pairs support in NEC plugin

Closes-Bug: #1230184
Change-Id: I84d4dda9f88f7bbb524e173b0274db013f69b7e3
Merge "Allowed Address Pairs support in NEC plugin"
Merge "Send proper exception info as expected by the neutron client"