Bug 138562 Splite extendsexcpetion to seperate sub class
The suggestion is Parent invalid Cycle extends Different type extends Not top level extends
model/org.eclipse.birt.report.model/src/org/eclipse/birt/report/model/api/validators/ElementExtendsValidator.java
* 
	 * @return the validator instance
	 */
	public static ElementExtendsValidator getInstance( )
	{
		return instance;
	}
	/**
	 * Validates the element reference value can refer to an actual element.
	 * 
	 * @param module
	 *            the module
	 * @param element
	 *            the element to validate
	 * @return error list, each of which is the instance of
	 *         <code>SemanticException</code>.
	 */
	public List validate( Module module, DesignElement element )
	{
		List list = new ArrayList( );
		if ( !StringUtil.isEmpty( element.getExtendsName( ) )
				&& element.getExtendsElement( ) == null )
		{
			list.add( new ExtendsException( element, element.getExtendsName( ),
					ExtendsException.DESIGN_EXCEPTION_NOT_FOUND ) );
		}
		return list;
	}
}