Added helpful error message to 500 page

I've added an error message to the 500 error page that will let the user know what to do if error messages have been disabled.

I've had to duplicate the message across both `NancyRazorErrorView.cs` and `DefaultStatusCodeHandler.cs` as I couldn't think of a better place to put it. If this is a problem and you rather it go somewhere else please let me know