Specify the errors given when Data API is inaccessible due to exclusive access given to another app instance

[phoglenix]

From the explainer:

It might not be possible for an operating system to provide simlultaneous access to the lock screen data store from both (lock screen and unlocked) instances of the app, as they could be running in isolated processes. In that case, the API methods could reject their promises while exclusive access is given to the other instance of the app.

We should specify what error will be used to reject promises in this situation, so developers can handle them appropriately.