Annotation Tools: Adding Try-Catch for anonyous_student_id issue in staging

@sarina I also changed the variable name from user to user_email for image annotation. Someone from edX in a previous PR asked me to change that previously and I'm assuming it was before the image annotation got added and it wasn't fixed in the image too. 

Should fix the issue in staging where ```self.user_email = self.runtime.get_real_user(self.runtime.anonymous_student_id).email``` threw an error where ```get_real_user``` returned None for some strange reason.
Why are you removing the default for `self.user`?