Error handling with webview_eval()

Evaluating JavaScript can fail for any number of reasons. The big one I ran into today is that when the browser's JavaScript thread is blocked by the debugger (though it could be blocked because it has a CPU heavy workload) `eval()` will silently fail to execute the code. This happens consistently on macOS.

I need a way to identify these kinds of errors for data resilience and application reliability. A quick look at all of the backends reveals they all support status reporting, either synchronously or asynchronously. (mshtml seems to be the only backend with a synchronous result.)

A channel or some other async feedback mechanism for error handling would be fine.

My workaround for this is probably going to be RPC over HTTP/Websocket.