Fixed crash when no SD Card is present

* Check for SD Card improved. We now not only look for mounted media but
  also make sure it's not READ_ONLY. The new function name reflects
  that.
* Added the check in MainMenuActivities onCreate() so we display an
  error message and quit gracefully instead of crashing.

I tried to write a test for this using PowerMock + Mockito but did not
succeed. In fact, I did not get to the point where I could even compile
with these libraries added. Android Support was added to Mockito in
1.9.5 but it seems there is no ready-to-use PowerMock package for
Android. I encountered various errors while trying to make a runnable
package.
The other approach would be to rewrite the functions with e.g.
Dependency Injection so we can insert e.g. a MockEnviroment to the Utils
function or similar approaches.
In the end, I decided to not do test-first in this case so this critical
issue can get closed and find a way to test it later on.
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.
Merge branch 'master' into SDCardFix#40
Merge pull request #136 from Catrobat/SDCardFix#40

Fix crash on App startup when no external storage is available