Fall back to IE if default browser can't be opened

[hartez]

The Dropbox authentication URL is opened using Process.Start(url), which should open the URL in the default browser. But it's possible for this to fail if the default browser configuration is broken (e.g., the registry not getting update if the default browser is uninstalled).

Need to add some error handling around this which falls back to IE if the default browser can't be opened.