FromParts<Protocol>::Rejection must implement std::fmt::Display for user-defined parameters passed to operation handlers

[drganjoo]

An operation handler can accept user-defined data types as long as they implement FromParts<Protocol>. The trait has an associated type Rejection. Whatever data type is set for Rejection must now implement (at least) std::fmt::Display. Additionally, it is recommended to implement std::error::Error for that type.

This requirement is necessary to help us write error log entries when the parameter that the operation handler expects fails to be
constructed from the request parts.

As an example of implementing std::error::Error, you can refer to the RuntimeError enum.