Add ExceptionHandled to ExceptionContext

[rynowak]

https://msdn.microsoft.com/en-us/library/system.web.mvc.exceptioncontext.exceptionhandled(v=vs.118).aspx#P:System.Web.Mvc.ExceptionContext.ExceptionHandled is missing from ExceptionContext. Our other 'after' context classes have it.

We should add this since this is the place you're most likely to use it.

[dougbu]

@ryanbrandenburg is this Done?

[ryanbrandenburg]

Yes, it's done. Thanks for the reminder.