Handle context freeze in the Mobile app

If a context is frozen, most WS calls will return an exception. E.g. the WS mod_assign_get_submission_status returns required_capability_exception.

Currently we have 2 problems in the app:

1. If some information is cached, the app will display the cached in this case.
2. If the information isn't cached, we'll display an error and, in some cases, some data will be displayed in the screen. E.g. in assignment we display an error and the screen displays a header "Submission status". In web, no error is displayed and the user only sees a blank screen, we should do the same as web.

We need to check all or most cases to make sure they all behave the same.