Bug 148951 Wrong error msg when set nonexisted col as group key
Description: Wrong error msg when set nonexisted col as group key Steps to reproduce: 1. Insert a table, bind it to dataset 2. Add a group, input abc as group key which is invalid. 3. Preview Expected result: Friendly error msg to tell user the group key is wrong. Actual result: + Group key "abc" in group 1 cannot depend on column bindings defined in the group lower than itself. data.engine.InvalidGroupKey ( 1 time(s) ) detail : org.eclipse.birt.data.engine.core.DataException: Group key "OUNTR" in group 1 cannot depend on column bindings defined in the group lower than itself.
data/org.eclipse.birt.data/src/org/eclipse/birt/data/engine/impl/ExprManagerUtil.java
}
	}
	/**
	 * 
	 * @param startNode
	 * @param candidateNode
	 * @return
	 * @throws DataException 
	 */
	private void isValidNode( Node startNode, Node candidateNode ) throws DataException
	{
		
		Object[] nodes = startNode.getChildren( ).toArray( );
		for ( int i = 0; i < nodes.length; i++ )
		{
			if ( candidateNode.equals( nodes[i] )
					|| startNode.equals( nodes[i] ) )
			{
				throw new DataException( ResourceConstants.COLUMN_BINDING_CYCLE,((Node)nodes[i]).getValue( ));
			}
			else
			{
				isValidNode( (Node) nodes[i], candidateNode );
			}
		}
	}
	/**
	 * 
	 * @param source
	 * @return