[sda] Consistent handling of messages

[dbampalikis]

As a developer
I want to have a consistent way of handling the messages and their errors (nacking, rerouting, sending to error queue etc)
In order to be able to trust the system

There is an effort in this document to map all the cases that can go wrong in the sda-pipeline, in order to handle them properly.

Rules:

• On all database errors, the message should be Nack'ed and requeued.