Improving "Session ID is null" error message

Signed-off-by: Luke Inman-Semerau <luke.semerau@gmail.com>

Author: sevaseva

java/client/src/org/openqa/selenium/remote/HttpCommandExecutor.java

@@ -273,7 +273,7 @@ public Response execute(Command command) throws IOException {
       }
       if (!GET_ALL_SESSIONS.equals(command.getName())
           && !NEW_SESSION.equals(command.getName())) {
-        throw new SessionNotFoundException("Session ID is null");
+        throw new SessionNotFoundException("Session ID is null. Using WebDriver after calling quit()?");
       }
     }