Support converting unhandled errors to specific types

[oguzkocer]

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.

[crazytonyli]

we can add some helpers to parse them to some known types.

This seems the most convenient way to do it. It looks like Rust errors can be exported as if they are uniffi::object (see this example code), which means we can hide Rust generic in them. That's probably something we can explore in the future if needed.

[jkmassel]

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

IMHO WpApiError::ResponseParsingError should probably be a global "we couldn't parse the response that came back" and WpApiError::UnknownError should be "this is a valid WPError object, but we don't recognize it from Core"