JavascriptAsyncMethodHandler::Execute - Add extra logging

- Add extra logging
- Fix formatting
- Add reference to relevant github issue

Follow up to #2446

Author: amaitland

CefSharp.BrowserSubprocess.Core/Async/JavascriptAsyncMethodHandler.cpp

@@ -27,8 +27,15 @@ namespace CefSharp
 
                 //this will create a promise and give us the reject/resolve functions {p: Promise, res: resolve(), rej: reject()}
                 auto promiseData = promiseCreator->ExecuteFunctionWithContext(context, nullptr, CefV8ValueList());
-				//when refreshing the browser this is sometimes null, in this case return
-				if(nullptr == promiseData) return true;
+                
+				//when refreshing the browser this is sometimes null, in this case return true and log message
+				//https://github.com/cefsharp/CefSharp/pull/2446
+                if (promiseData == NULL)
+                {
+					LOG(WARNING) << "JavascriptAsyncMethodHandler::Execute promiseData returned NULL";
+
+                    return true;
+                }
 
                 retval = promiseData->GetValue("p");