Returns true if the Deferred object is in the rejected state, meaning that either deferred.reject() or deferred.rejectWith() has been called for the object and the failCallbacks have been called (or are in the process of being called).

Note that a Deferred object can be in one of three states: pending, resolved, or rejected; use deferred.isResolved() to determine whether the Deferred object is in the resolved state. These methods are primarily useful for debugging, for example to determine whether a Deferred has already been resolved even though you are inside code that intended to reject it.