Enroling into an already enroled course does not give a meaningful error

[abhijeetviswa]

If a user enrols to a course on the website and then attempts to enrol from the app once again, the app does not create a meaningful error. Ideally, the user should be notified about the possibility of already being enroled or the course list should be fetched and checked to see if the user is already enroled.

Alternatively, the course list should be updated in MyCoursesFragment#onCreate, like how course content is updated in CourseContentFragment, and ForumFragment.