code: function waitForReady (method) {
    return function fetchSync_readyWrapper (...args) {
      if (hasStartedInit) return ready.promise.then(() => method(...args))
      throw new Error('initialise first with fetchSync.init()')
    }
  }
docs: Wrap a function to wait for the application to be initialised
  (comms channel with service worker is open) before executing.
  @param {Function} method
  @returns {Function}