Uniquify horizon messages returned in a single response

A network operation can fail because of different
reasons - yet in many places just one error message
is provided. Combined with too broad exception clause,
and incorrect assumptions on the reasons of failure
(e.g. Neutron service being unavailable causes all
other sorts of errors like inability to Allocate IP
or Associate it) this leads to multiple errors when just one
would suffice. The fix aims to provide sensible error
messages in case the network service is down. This includes
returning only unique messages in a single response.

Partial-Bug: #1301374
Change-Id: Id6c620ca51f7703f35c0c172e39fdf237fa42278
Co-Authored-By: Timur Sufiev <tsufiev@mirantis.com>
Merge "Uniquify horizon messages returned in a single response"