fix(plugins): aws consume plugin transaction management

[wssheldon]

Explicitly handle rollback on exceptions in signal_service.create_signal_instance. Exceptions are not raised in the AWS consume plugin, exceptions are caught, logged, and the loop continues (ultimately ending up in the SQS DLQ, if configured). The outer context manager in cli/consume never calls rollback as a result of this.

Here is the documentation on begin_nested since it's not easy to find for our current version: https://docs.sqlalchemy.org/en/13/orm/session_transaction.html#session-begin-nested