* made NFW to use runtime callstack rather than callstack from fake exception

also, made WatsonReporter from VS and ServiceHub to share more implementation

* PR feedback.

rather than using null exception which is taken cared by NFW, we explicitly throw exception to have callstack so that it can show up in perfview as well.

* centralize who throws exception if given exception is never thrown.

also, explicitly add runtime callstack in one of reserved slot to make NFW bucketting better.

* fixed tests and added empty callstack test