Travis: Start xvfb for GUI tests.

Tested on #498.
Closes #499.
Catch authentication error with https.

Change the catch logic to throw a MissingCertificateKraken in the case of authentication failure ("System.IO.IOException: The authentication or decryption has failed.").
Show error message for missing certificate in the GUI.
Merge remote-tracking branch 'pjf/nunit_gui_tests' into release-v1.2

* pjf/nunit_gui_tests:
  Travis: Start xvfb for GUI tests.
Merge remote-tracking branch 'mgsdk/certificate-fix' into release-v1.2

* mgsdk/certificate-fix:
  Show error message for missing certificate in the GUI.
  Catch authentication error with https.