Add `deleteClass` method for `CreateCtrl`

* I added `StringResource` and `Utils` as dependencies for `CreateCtrl` controller to make use of custom exceptions.
* I added a custom exception named `ClassNotFoundError` which extends `RangeError`.
* I added a new method `deleteClass` to `$scope` and this method deletes a class object from `classCart` and `storedClassCart`, then it saves `storedContext` to cookie.
* I added a string resource for the error message for `ClassNotFoundError` exception.
* I added a utility function which extends an (parent) error type to another (child) error type, and returns the child error type.
* I fixed the test case 'should delete a class entity from class cart...' to work correctly.