Hiding warning message

- When starting Yast in interchangable (GUI) interfaces, Yast used to
  report is as warning. On systems, where we don't have GTK/Qt, it
  doesn't hurt to change one to another, but a warning message confuses
  users and makes them report it as error (bnc#861807)
Changes and version
Error is reported if user explicitely wanted the GUI he hasn't got
Merge pull request #254 from kobliha/hide-falling_back_to-message

Hiding "falling back to..." message