Add anyhow::Error example

[TmLev]

Made a simple example showing how to integrate anyhow::Error with rspc's typed error handling.

An unfortunate quirk: anyhow::Error cannot be easily converted into std::error::Error + Send + 'static. dtolnay suggests using thiserror to create a transparent wrapper.

Resolves #215.

cc @oscartbeaumont

[vercel]

@TmLev is attempting to deploy a commit to the Oscar Beaumont Team on Vercel.

A member of the Team first needs to authorize it.

[oscartbeaumont]

Thank you!