a custom checked exception is wrapped as OperationException

Steps to Reproduce

1. configure erro-page in web.xml to catch a specific exception

   	<error-page>
   		<exception-type>support.error.MyCheckedException</exception-type>
   		<location>/error.zul</location>
   	</error-page>
   

2. throw a checked exception that impelments Expectable (like attached MyCheckedException.java) in a zul like:

   <zk>
   <zscript><![CDATA[
   throw new support.error.MyCheckedException();
   ]]></zscript>
   </zk>
   

Current Result

it's wrapped as OperationException, so users can't handle it directly by its class name. it doesn't redirect to error.zul

Expected Result

redirect to error.zul

Debug Info

• According to document, https://www.zkoss.org/wiki/ZK_Developer%27s_Reference/UI_Patterns/Error_Handling,

  f you want to handle your own exceptions you can implement the Expectable on your exception type. Exceptions implementing this interface will not be wrapped and can be handled using the <exception-type> element directly.

  Current implementation doesn't fit this description.