`ReadError` and `WriteError` should support custom errors

[qkniep]

Currently, there seems to be no support for throwing custom error types. This is useful for any type that needs to do arbitrary verification during (de)serialization, e.g. a signature type that needs to check whether the point is on the elliptic curve.

There should probably be a way to throw custom ReadErrors and WriteErrors, e.g. by using map_err() on some error type within the implementations of SchemaRead::read() and SchemaWrite::write(). This is analogous to this use case in serde: https://serde.rs/convert-error.html