add exposeInvocationInterceptor exception msg

• because ExposeInvocationInterceptor use ThreadLocal to expose MethodInvocation in current Thread, thus ExposeInvocationInterceptor.currentInvocation() that invoking in other thread will throw Exception.
• some spring users face the problem cann't know the reason. for example:
  https://stackoverflow.com/questions/7147031/spring-aspect-fails-when-join-point-is-invoked-in-new-thread?r=SearchResults

Thank you.