Support converting unhandled errors to specific types

We have been able to convert all error types to `WpApiError` for WordPress REST API endpoints. However, if a site returns a custom error, we can't parse it and will return it as `WpApiError::Unknown`. The client can further parse the response using the `WpApiError::Unknown::response` field, but we can add some helpers to parse them to some known types.

We also probably don't need both `WpApiError::ResponseParsingError` & `WpApiError::UnknownError`, what do you think @crazytonyli?

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One alternative we could consider is to allow each endpoint to return its own error type. I think this would work very well if this was a standalone Rust project, because we could use generics and trait bounds to still have some central handling. However, it may prove difficult for a `uniffi` project, because we are not allowed to use generics in the bindings and we can't return a trait bound object if I remember correctly.

Another alternative would be to add some WordPress.com specific error types, but that won't be a generic solution that'd allow working with custom plugins in the future.