ColorConverter exception prevention

I have made a small fix that prevents the ColorConverter from throwing several exceptions during startup when trying to parse default border color. When you enable "break-on-exception" to catch your own bugs it can be somewhat annoying to have a library throwing exceptions during normal startup.

Also I fixed a small typo.

Apart from that I will just say thanks for the great library! It fills a big hole in the WPF widget with the standard Microsoft WebBrowser widget being little more than a WinForms control in disguise.