Use `MappingError` in Marshal extensions

[olejnjak]

As we do not like the force cast of error, we should use generic MappingError also for Marshal extensions (in the same way we do it for Codable)

https://github.com/AckeeCZ/ACKReactiveExtensions/blob/d7ced4c5e685a56099187d31542a302f9109a0fa/ACKReactiveExtensions/Marshal/MarshalMapping.swift#L61