We discussed this in years gone past but I can't remember where we landed.

The default impl of std::error::Error returns None so there are two ways to implement the trait for an error type that does not have an inner error.

Less explicit

#[cfg(feature = "std")]
impl std::error::Error for UnknownDenominationError {}

More explicit

#[cfg(feature = "std")]
impl std::error::Error for UnknownDenominationError {
    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> { None }
}

We have a ton of both, should be pick one and be uniform?