message: Workaround the JS serialisation issue for errors.
  Currently, getting a PolyglotProxy error as a JavaScript string is being passed.
  This works around the problem by concatenating an empty string.
diff: @@ -17,4 +17,7 @@ error_preprocessor x =
  stack_trace = x.get_stack_trace_text.if_nothing "" . split '\n'
  full_message = message + if stack_trace.length > 1 then " (" + stack_trace.at 1 . trim +")" else ""
  JS_Object.from_pairs [['kind', 'Dataflow'], ['message', full_message]] . to_json
  - if result.is_error then result.catch else ok
  +
  + ## Workaround so that the JS String is converted to a Text
  + https://www.pivotaltracker.com/story/show/184061302
  + "" + if result.is_error then result.catch else ok
  