fix backtraces in watchman.ml
fix backtraces in watchman.ml

Summary:
use lwt_ppx instead of ppx_let, since lwt_ppx inserts some `%reraise` calls to preserve backtraces, while `ppx_let` doesn't.

before, we got backtraces like this, which jump from watchman.ml:390 into Lwt internals, missing which other promises called watchman.ml:390.

```
Unexpected error in watch_project: Unix.Unix_error(Unix.EBADF, "read", "")
Raised by primitive operation at file "src/unix/lwt_unix.cppo.ml", line 235, characters 14-31
Re-raised at file "/home/mroch/flow/src/hack_forked/watchman/watchman.ml", line 390, characters 4-176
Called from file "src/core/lwt.ml", line 2048, characters 23-28
Called from file "src/core/lwt.ml", line 1214, characters 10-18
Called from file "src/core/lwt.ml", line 1280, characters 17-21
Called from file "src/core/lwt.ml", line 1316, characters 4-103
Called from file "src/core/lwt.ml", line 1998, characters 12-55
Called from file "src/core/lwt.ml", line 1214, characters 10-18
Called from file "src/core/lwt.ml", line 1280, characters 17-21
Called from file "src/core/lwt.ml", line 1316, characters 4-103
Called from file "src/core/lwt.ml", line 1945, characters 12-71
Called from file "src/core/lwt.ml", line 1214, characters 10-18
Called from file "src/core/lwt.ml", line 1280, characters 17-21
Called from file "src/core/lwt.ml", line 1316, characters 4-103
Called from file "src/core/lwt.ml", line 1891, characters 12-55
Called from file "src/core/lwt.ml", line 1214, characters 10-18
Called from file "src/core/lwt.ml", line 1280, characters 17-21
Called from file "src/core/lwt.ml", line 1316, characters 4-103
Called from file "src/core/lwt.ml", line 1390, characters 37-76
Called from file "array.ml", line 93, characters 31-48
Called from file "src/core/lwt_sequence.ml", line 128, characters 31-47
Called from file "list.ml", line 110, characters 12-15
Called from file "src/unix/lwt_engine.ml", line 357, characters 4-63
Called from file "src/unix/lwt_main.ml", line 33, characters 4-78
Called from file "/home/mroch/flow/src/common/lwt/lwtInit.ml", line 129, characters 4-135
Called from file "/home/mroch/flow/src/hack_forked/utils/sys/daemon.ml", line 150, characters 4-20
Called from file "/home/mroch/flow/src/hack_forked/utils/sys/daemon.ml", line 272, characters 4-26
Called from file "/home/mroch/flow/src/flow.ml", line 106, characters 4-31
```

Reviewed By: samwgoldman

Differential Revision: D28009850

fbshipit-source-id: 91c337caac36c25ee7edc646a3863700dd2c5012

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