fix(casting): properly follow an unpopulated node

[michaelfig]

closes: #8601

Description

If the multistore entry being followed was not yet created, the @agoric/casting follower would fixate on the current block height, and not try advancing the cursor to the latest entry. This fix keeps trying the latest entry to see if it has any new contents.

Also add follow error handling to agoric follow.

Security Considerations

n/a

Scaling Considerations

n/a

Documentation Considerations

n/a

Testing Considerations

Manually tested in UIs.

Upgrade Considerations

n/a

[kriskowal]

(Also looks good to me.)

What kinds of error cells occur? Do some of them suggest that the stream should terminate? Errors emerging from an async iterator usually imply termination.

[michaelfig]

What kinds of error cells occur?

Deserialization errors, or problems interacting with a given RPC node.

Do some of them suggest that the stream should terminate?

Not usually, as auto-failover and round-robin behaviour can be part of the Leader that is being consumed by the iterator. They should be surfaced for the application to deal with, but continuing to produce values is legitimate behaviour.

Errors emerging from an async iterator usually imply termination.

Termination is a legitimate application response. And unrecoverable internal errors in the iterator code would cause unavoidable termination.