Async OnException

[akashlal]

Currently the OnException callback isn't async. It cannot be because it is called in a when clause, as the thrown exception is being propagated up the call stack. An alternative solution is to have another (async) callback that is invoked when the user decide to handle the exception. This is prototyped on the branch AsyncOnException (needs more tests).

[akashlal]

Alternative is to use throw instead of "throw ex". Page 1 and Page 2