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.
Fixed format string in GUI/Program.cs
Now the GUI will display a some information in the case of unhandled exceptions. May help with bug reports.
Catches DownloadErrorsKrakens gracefully

Closes #368.
Merge pull request #514 from RichardLake/DisplayExecptionInfo

Fixed format string in GUI/Program.cs
Merge pull request #520 from Ippo343/master

Catches DownloadErrorsKrakens gracefully
Merge pull request #503 from pjf/nunit_gui_tests

Travis: Start xvfb for GUI tests.
Merge pull request #504 from mgsdk/certificate-fix

Certificate fix