Be more userfriendly when the application cannot retrieve user data

[jlnknz]

For now, if the UserManager fails in getUserData() because of IOException, the application crashes as the UserInstance and UserPreferencesInstances objects are considered mandatory. We could do something a bit more userfriendly (display a dialog?).

• If we throw FocusMissingResourceException in getUserData(), then the EntrypointActivity will display a remediation dialog (=try again).
• In FocusAppLogic#sync(), we can also catch the same exception and return gracefully without crashing (this is already done, but requires consistency with the previous point)

[jlnknz]

Done.

[jlnknz]

Done.