Expose fields (or add access methods) of MessageNotFoundErr

[stephenafamo]

With errors.As, it is now possible to detect the error, but since the fields are private, there is no way to extract the structured details without resorting to string parsing.

[nicksnyder]

What is your use case for wanting access to the underlying fields?

[stephenafamo]

For now, I am doing structured logging and would prefer to have the language and message key as fields.

In the future, I intend to send custom alerts to ease the translation flow.

[stephenafamo]

Any intention of exposing this? 🙏🏾

[nicksnyder]

I don't have a strong objection. If you want to open a PR I will review it.