Document our Rust coding standards for error/failure types
Every crate which returns
Result<T, E>s or
Option<T> anywhere in its public interface should have an
errors.rs module containing error types which implement either
Debug. See the
errors.rs addition to the
protover crate from #24031 (moved) for an example.
In the future, when failure is 1.0.0, we should also require
::failure::Fail for making errors easier to work with between crates.