async function asyncCallback(df, callback) {
    try {
      await callback(df);
    } catch (err) {
      df.error(err);
    }
  }
Invoke and await a potentially async callback function. If
an error occurs, trap it and route to Dataflow.error.
@param {Dataflow} df - The dataflow instance
@param {function} callback - A callback function to invoke
  and then await. The dataflow will be passed as the single
  argument to the function.