make gRPC buck build script fail fast on error

- See comment in: https://gerrit.onosproject.org/14531
  for more details about the problem.

Change-Id: I7ef3aa92dfa5116a077f3d6878856b555cfda93c
diff --git a/incubator/protobuf/models/BUCK b/incubator/protobuf/models/BUCK
index 99619ed..81a9b25 100644
--- a/incubator/protobuf/models/BUCK
+++ b/incubator/protobuf/models/BUCK
@@ -23,6 +23,8 @@
 grpc_jar(
     name = 'onos-incubator-protobuf-models-proto',
     deps = GRPC_DEPS,
+    # FIXME should not be manually building absolute path.
+    # Come up with glob(..) equivalent in BUCK which can handle directory.
     proto_paths = ["$ONOS_ROOT/incubator/protobuf/models/src/main/proto"]
 )