Throw an exception when you forget to register a BatchingStrategy

I've noticed when using parseq-batching that if I forgot to register my BatchingStrategy that the Task will hang indefinitely. It would be better to throw an exception like "unknown batching strategy" in this case instead.

Start by tracing the parseq-batching registration path and how a Task resolves its BatchingStrategy. Check the behavior when the strategy is missing, then add coverage for the unregistered case. Done means the Task fails with an "unknown batching strategy" exception instead of hanging indefinitely.

Written by the indexing model from the issue text.