Add custom exceptions for jsonrpc lookups that return none

[njgheorghita]

What was wrong?

Rather than simply raising a ValueError when jsonrpc lookup calls returned None, raise a custom exception with better info.

re. comment here

Cute Animal Picture

[pipermerriam]

Does this ValueError come from an error type response from the JSON-RPC server? Is this client specific behavior (like geth returns an error response and parity returns empty)? Struggling to remember how all of this works.

I ask because it feels a little worrisome to be blindly converting ValueError -> ThingNotFound as this would be an easy way for our exception messaging to be wrong in a case where the ValueError means something else but we're blindly reporting it as a ThingNotFound.

Is there a way we can be more precise with this?

[njgheorghita]

@pipermerriam The ValueError comes from here which I implemented in the previously-related pr to be raised everytime a jsonrpc call returns None - off the top of my head, I don't think it's client-specific behavior meaning that it would probably require some more complexity to have a more precise error message, but if you 👍 I'll dig deeper and see about improving the error reporting

[pipermerriam]

I think we should drop at internal raising of the ValueError and do the None detection within each of the individual methods. The current ValueError based approach has a strong potential for false positives, and completely disallowing None returns from JSON-RPC calls globally probably isn't a tenable API (sorry I didn't catch that previously).