Make Ember.observer work with the function as the last argument. Support the old style with a deprecation notice.
Update tests with the new style for Ember.observer.
Update Ember.immediateObserver and Ember.beforeObserver to match the new Ember.observer style.
Only soft-deprecate the old Ember.observer style.
User Ember.Error for errors.
Merge pull request #3568 from machty/loading-substates

[FEATURE] Loading/error substates
Merge pull request #3590 from gunn/master

Change Ember.observer and friends to take function as the final argument
Update docs to reflect that Bundler is needed for the build process
Merge pull request #3612 from DuncanMacWeb/add-bundler-to-docs

Update docs to reflect that Bundler is needed for the build process
[FEATURE ember-router-loading-error-substates] flaggify substates

Much of the internal implementation for the Router had to change
to allow for substates, hence my reluctance to throw feature flags
on the previous substates commit, but we do in fact want/need
feature flags, so here they are. When the feature flag is disabled,
legacy (inconsistent/kind of broken) LoadingRoute behavior is
preserved.