[proc macros] force proc macro api to return `Result` (#435)

* rewrite me

* require proc macro API to return result

* send unknown message when error is not CallError

* show example that auto cast StdError doesn't work

* register_*_method Into<Error>

* clippy

* replace generic errors with anyhow::Error

* fix nits

* example that anyhow::Error in register_method works

* CallError: add missing From impl

* [types]: add helper methods for Error types

The rationale is to make it possible for users to either use anyhow::Error or use the helper methods.

* fmt

* Revert "register_*_method Into<Error>"

This reverts commit 33b4fa28730b72647ba150659d3c0ab1937e524a.

* add better comment

* fix nit