Add getter for the stanza associated with the exception

Fixes SMACK-916

smack-core/src/main/java/org/jivesoftware/smack/XMPPException.java

@@ -120,6 +120,16 @@ public abstract class XMPPException extends Exception {
             return error;
         }
 
+        /**
+         * Gets the stanza associated with this exception.
+         *
+         * @return the stanza from which this exception was created or {@code null} if the exception is not from a
+         * stanza.
+         */
+        public Stanza getStanza() {
+            return stanza;
+        }
+
         /**
          * Get the request which triggered the error response causing this exception.
          *