Publish unhandled errors to telemetry source

[Eilon]

Publish this:

1. Unhandled errors - wherever in there's a catch that doesn't rethrow (same place that stuff is logged right now, probably)
   • Need to do this in both DeveloperExceptionPage and ErrorHandler

Example in MVC: https://github.com/aspnet/Mvc/blob/dev/src/Microsoft.AspNet.Mvc.Core/Actions/MvcRouteHandler.cs#L73-L78

[Eilon]

Related: aspnet/Hosting#354

[JunTaoLuo]

Just to confirm, the only unhandled exception that needs to be written to telemetry is at DevelopperExceptionPageMiddleware.cs#L86 where we just create an exception page. All other instance of Invoke in DevelopperExceptionPageMiddleware.cs will end up throwing an exception that bubbles up to hosting.

Similarly ExceptionHandlerMiddleware.cs will throw unhandled exceptions up to hosting or handle it at ExceptionHandlerMiddleware.cs#L65

cc @Tratcher @Eilon

[Tratcher]

Correct, you only need telemetry for app exceptions that are not re-thrown.