Convert MidoApiConnectionError to a subclass of NeutronException

The python client may throw a MidoApiConnectionError exception if
the midonet-api is unreachable. In this case for neutron not to
crash, the neutron midonet plugin must catch this exception and
convert it to a subclass of NeutronException. In this case it
will raise a MidonetApiException(neutron.NeutronException)