Certificate fix

I had trouble getting the CKAN GUI to work on linux, or rather return expected results. After digging around for a while, I see that the error is related to a missing certificate on new mono installs on linux. I can see the code attempts to catch this case, but it does not appear to succeed.

I have changed the catch logic and added a separate check case in the GUI to display the error message in MissingCertificateKraken.

Note: I have not worked with C#/.Net in a long time, and have never been involved in C#/.Net GUI development before. I haven't really been a part of GitHub collaborations before either, so any critique is welcome.