Hardened MainMenu against FCs without ext. storage

* Added checks in onResume() and onDestroy() that can cause FCs in rare
  cases when there is no external storage available
* projectManager member could be null because it was never initialized
  when there was no external storage available at the start of the app.
* Strings modified to more accurately reflect what happens (too
  technical?)
* App doesn't just System.exit(0) anymore but simply moves in the
  background. This was done to retain the Android paradigm of not
  killing your app manually, even though this is an unrecoverable state.