No error message displayed to user when learning unit / app detail fails to load

[mikedawson]

Describe the bug
When a learning unit (eg lesson) or app fails to load (e.g. because of errors by the developers, or connectivity, etc) no clear error message is shown to the user.

To Reproduce
Steps to reproduce the behavior:

1. Add an app open a list of lessons from the app
2. Put phone in flight mode
3. Click on an item from the list

Expected behavior
Clear error message showing why content could not be loaded.

Screenshots
Pending

Smartphone (please complete the following information):
Android: all versions

Additional context
Add any other context about the problem here.