Currently, if a channel is detected as closed in a nested function call, we propagate either false or None, and in those cases just return; in the parent function.

Examples:

• stream_next_batch
• stream_block_range

Ideally, the code should be self-explanatory and/or force us to handle channels closing in the parent function.
Consider:

• using a new error type
• custom enum ChannelState::Open / Closed
• always propagating errors to the top-most parent function, that then streams the errors.