SLING-4219  Prevent IllegalStateException if SlightlyParserErrorListener is called with null RecognitionException

Note: The actual problem is the SlingException(String, Throwable) constructor
   which calls initCause if the Throwable is null. This should be fixed as well.
   
(Applying patch by Radu Cotescu, thanks for providing the patch!)

git-svn-id: https://svn.apache.org/repos/asf/sling/trunk@1643248 13f79535-47bb-0310-9956-ffa450edef68