Added error handling to multiple controllers (#966)

* Added error-checking to annotations_controller

* Added error-handling to problems_controller

* Added error checking to courses_controller.rb

* Added error handling to submissions_controller

* Removed unnecessary error handling, fixed typos

* Removed unnecessary error handling in problems_controller

* Removed trivial error handling in courses_controller

* Removed trivial error checking in annotations_controller

* removed a few extraneous error handlers