Work around bug #2262 - Crash on starting when configuration file is corrupted.

I felt this problem needed some kind of dialog box to appear in order to alert users to what is happening as when the problem occurred for me i tried all sorts of things to get GitExtensions working again. Knowing at least where to look would have saved me a few hours - so hopefully this will help someone else save some hours.

This exception occurs before NBug is loaded so you wouldn't normally receive the bug reports.

I tried to stick to the style guidlines - this is the first fix I've done so please let me know if I've done it wrong - and how to do it right.