I think this may be a bug in $q.reject (see https://github.com/angular/angular.js/blob/master/src/ng/q.js#L277):

If I chain a reject with a then but do not provide an errback callback function the $q.reject method blows up.

See this fiddle: http://jsfiddle.net/dWbWJ/.

I believe that both arguments to the promise.then method can be optional (or at least should be).
I think the $q.reject method should test for the errback before trying to call it.