Improve error handling in RPC package

[lthibault]

On several occasions I have found myself wanting to inspect RPC exceptions to control execution flow, in particular via functions such as errors.Is or errors.As. Currently, errors are collapsed when composed, making them difficult to inspect.

This PR is an incremental attempt at improving error composition in the code base. It exports the errors.Error type, enhances it with an Unwrap() method to make it compatible with the standard library errors package, and replaces %v formatting directives with %w and fmt.Errorf.

In addition to making errors more interpretable for the user, the resulting code is much more readable IMHO.

[lthibault]

@zenhack I just rebased against main and double-checked that the errors string was properly formatted. Are you ok to merge this?

[lthibault]

@zenhack Re your question about quotes (can't seem to reply for some odd reason):

1. The Sprintf call made things readable enough IMO without the strconv.Quote call
2. Error message is still quoted (quotes are inline, negating the need for strconv.Quote)
3. An error value can't be passed to strconv.Quote directly. Calling the Error() method seemed verbose.

tl;dr: it was a judgement call about readability. Outputs should be identical, though, and we can revert if you prefer.

[zenhack]

strconv.Quote is more than just return "\"" + s "\""; it escapes things inside the string as well.

But you can also use a %q format specifier, which would keep the full escaping while also being more concise. It also works with errors.

[lthibault]

strconv.Quote is more than just return "\"" + s "\""; it escapes things inside the string as well.

Oh, I didn't realize that. Makes sense.

But you can also use a %q format specifier, which would keep the full escaping while also being more concise. It also works with errors.

This sounds like our winner. Will push changes shortly.