postError method is used to send back an error message to the app that made the activity request.
- A string describing the error. This string will be used by the system to create a DOMError object.
Web Activities is not part of any specification. However, it has some overlap with the proposed Web Intents specification. Mozilla actually proposed Web Activities as a counter proposal to Web Intents. For more information about this, see discussion on the Web Intents Task Force ML.