Error streaming: Log more when failing to get field files from watc…

…hman response.

Summary:
Some user has reported this error, so logging more, since I couldn't get their logs:
```
Watchman failure.
Hh_json_helpers.Jget.Parse("files")
Raised at Hh_json_helpers.Jget.get_exn in file "fbcode/hphp/hack/src/utils/hh_json/hh_json_helpers.ml" (inlined), line 55, characters 14-31
Called from Hh_json_helpers.Jget.array_exn in file "fbcode/hphp/hack/src/utils/hh_json/hh_json_helpers.ml" (inlined), line 117, characters 18-35
Called from Hh_json_helpers.Jget.string_array_exn in file "fbcode/hphp/hack/src/utils/hh_json/hh_json_helpers.ml", line 123, characters 4-22
Called from ClientCheckStatus.watchman_get_raw_updates_since.(fun) in file "fbcode/hphp/hack/src/client/clientCheckStatus.ml", line 324, characters 18-75
```

Differential Revision: D66300063

fbshipit-source-id: 1beadecbf3723e13c78a54092b8c721f2efd493e

hphp/hack/src/client/clientCheckStatus.ml

@@ -405,7 +405,14 @@ let watchman_get_raw_updates_since
     match files with
     | Error e ->
       let msg = Exception.to_string e in
-      Hh_logger.log "watchman parse failure: %s\nRESPONSE:%s\n" msg stdout;
+      let msg =
+        Printf.sprintf
+          "ClientCheckStatus: watchman parse failure. Expected field `files`.\n%s\nRESPONSE:%s\n"
+          msg
+          stdout
+      in
+      Hh_logger.log "%s" msg;
+      HackEventLogger.invariant_violation_bug msg;
       Lwt.return_error msg
     | Ok (json, files) ->
       let has_changed = ref false in