_process_streaming methods and robustness

[seanpearsonuk]

We have several _process_streaming methods for a number of services. Structurally, these are similar, so it seems natural to write a template method around these.

Is there some additional behaviour that could be enforced in such a template? Each function breaks out on StopIteration. But what about other exceptions? It's difficult to analyze the code end-to-end to predict all possible exceptions. But what's clear is that if exceptions are raised in the current set-up then the user is likely to be presented only with some low-level information. E.g., #1504 deals with an IndexError. We could handle exceptions generically in the template method and report the high level error (raise from).

We should also take care regarding the robustness of the subclass implementations, identifying any potentially unhandled exceptions that should not halt the functions.