Routing table dispatcher loses messages on Riak errors.

[hodgestar]

Unhandled errors in the routing table dispatcher fall through to default_errback (https://github.com/praekelt/vumi/blob/develop/vumi/dispatchers/endpoint_dispatchers.py#L56-L57). This logs the error and then acks the message to rabbit, causing the message to be marked and processed and logged. I propose that we implement a custom default_errback for the routing table dispatcher that logs the error and then re-raises it so that the message is not acked and returned to the Rabbit queue.

This is similar to what happened with the billing dispatcher recently (although that was addressed in a different way).

[hodgestar]

@jerith @rudigiesler @justinvdm -- thoughts?

[jerith]

Sounds reasonable.

[justinvdm]

+1, surprised that I haven't heard about this biting us before though.

[jerith]

We haven't had very many errors of this sort in the past.