Cef.h - Improve error message when calling method before Cef Initialized
Cef.h - Improve error message when calling method before Cef Initialized

- InitializedCheck renamed to ThrowIfCefNotInitialized
- ThrowIfCefNotInitialized added to more methods

Follow up to #2605

Livit/CefSharp,Livit/CefSharp,Livit/CefSharp,Livit/CefSharp