errors: export Status2TerrCode

[slai]

This PR exports the errors.Status2TerrCode function. It is currently unexported, and also unused.

I'd like to export this as it is useful for implementing custom error filters, for example, when using a Typhon client to talk to third-party APIs, it may be necessary to implement custom logic to deserialize the errors returned into Terrors. As part of that, it is common to translate the response HTTP status codes into Terror error codes.

In our codebase, I can see around 15 re-implementations of this mapping, so I think there is sufficient usage for this change.

Alternatively, I think there's an argument for this mapping to be in monzo/terrors, because a number of the usages I found actually have nothing to do with monzo/typhon, and it is just converting HTTP status codes returned by other clients into terrors to be returned.