[hack] Include original stack trace from Nonfatal_rpc_exception in master_exception
[hack] Include original stack trace from Nonfatal_rpc_exception in master_exception

Summary:
Top firing master exception is right now:

```
File "hphp/hack/src/utils/utils.ml", line 194, characters 2-8: Assertion failed
Raised at file "hphp/hack/src/server/serverCommand.ml", line 268, characters 11-57
Called from file "hphp/hack/src/server/serverMain.ml", line 187, characters 5-41
```

Not really actionable, and it's because we swallow the actual interesting stack trace.

Reviewed By: vassilmladenov

Differential Revision: D7593660

fbshipit-source-id: ae1d492af999cfbdadee464398a27dc25dda3259

mroch/flow,popham/flow,samwgoldman/flow,JonathanUsername/flow,samwgoldman/flow,samwgoldman/flow,facebook/flow,nmote/flow,samwgoldman/flow,jamesgpearce/flow,claudiopro/flow,facebook/flow,samwgoldman/flow,jamesgpearce/flow,facebook/flow,nmote/flow,claudiopro/flow,gabelevi/flow,jamesgpearce/flow,nmote/flow,nmote/flow,popham/flow,claudiopro/flow,AgentME/flow,nmote/flow,AgentME/flow,nmote/flow,jamesgpearce/flow,JonathanUsername/flow,JonathanUsername/flow,popham/flow,facebook/flow,gabelevi/flow,AgentME/flow,JonathanUsername/flow,jamesgpearce/flow,gabelevi/flow,mroch/flow,popham/flow,claudiopro/flow,gabelevi/flow,AgentME/flow,mroch/flow,mroch/flow,jamesgpearce/flow,JonathanUsername/flow,popham/flow,JonathanUsername/flow,jamesgpearce/flow,gabelevi/flow,popham/flow,mroch/flow,mroch/flow,JonathanUsername/flow,AgentME/flow,facebook/flow,facebook/flow,gabelevi/flow,samwgoldman/flow,claudiopro/flow,gabelevi/flow,mroch/flow,facebook/flow,samwgoldman/flow,popham/flow,nmote/flow