BridgeJS: Support throwing any error conforming to ConvertibleToJSException

[wfltaylor]

Introduce a new protocol ConvertibleToJSException and support throwing any error (as long as it is specified using typed throws) which conforms to this protocol. It is beneficial for @JS methods to be usable by non-JS consumers, but requiring the error type to be JSException makes this unachievable for methods which throw.

This PR also fixes an issue where the generated code for throwing methods doesn’t compile at all using Embedded Swift, since it uses String(describing:).

A potential alternative to this approach would be to emit code to support @JS annotated Swift Errors, which would bridge them to JavaScript. This is a bit more complex so I stuck with the more straightforward approach as a first attempt.

[MaxDesiatov]

It is beneficial for @JS methods to be usable by non-JS consumers, but requiring the error type to be JSException makes this unachievable for methods which throw.

I'm not sure I understand, can you elaborate and provide an example?

[wfltaylor]

It is beneficial for @JS methods to be usable by non-JS consumers, but requiring the error type to be JSException makes this unachievable for methods which throw.

I'm not sure I understand, can you elaborate and provide an example?

For example, someone might want to write cross platform SDK which is consumed by JavaScript on the web, but Java on Android, and C++ on the server. Currently, any throwing function would need to be wrapped to map any error to JSException to annotate it with @JS. Instead, if we could use a type which conforms to ConvertibleToJSException, the same method could be called from JavaScript code, Swift code, and any other suitable consumer.