Created Issue: WebPages and Razor should use Error helper methods [320]

WebPages and Razor currently use their own version of ExceptionHelper rather than Error.
This tasks entails the following:
1) Convert all calls to ExceptionHelper into calls to Error (in Common folder)
2) Deprecate ExceptionHelper
3) Convert all "throw new XyzException" into calls to Error.Xyz
4) Change the AssertEx.ThrowArgumentNullOrEmpty to use the same resource that Error.ArgumentNullOrEmpty uses so unit tests and common\Error agree