fix stack traces in WorkerController.call
fix stack traces in WorkerController.call

Summary:
i'm trying to track down exceptions like this:

```
Unhandled exception: Other_send_job_failure: Unix.Unix_error(Unix.EPIPE, "write", "")
Raised at Stdlib__map.Make.find in file "map.ml", line 137, characters 10-25
Called from Lwt_engine.select_or_poll_based#register_readable in file "src/unix/lwt_engine.ml", line 304, characters 8-36
```

`Other_send_job_failure` is only raised by `WorkerController.send`, which is called by `WorkerController.call`. `call` uses `Lwt.finally`, which doesn't record locations in its backtraces.

I don't know if this is *the* cause of the stackless exception, but it could be *one* cause.

Note that there's an `[%lwt.finally]` ppx syntax that (allegedly) does record locations. I unfortunately removed it from this exact callsite in D23351124 (https://github.com/facebook/flow/commit/1212836136f9fffa44ae1ca281c79328919afbc7)! but it's very weird, so let's just use `try%lwt` instead... the finally block is a one-liner.

Changelog: [internal]

Reviewed By: samwgoldman

Differential Revision: D35792572

fbshipit-source-id: 8854f3c583a2004eb54926dc238707ac570cb780

facebook/flow,mroch/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow