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/command/ExtendsException.java
/**
	 * The parent element does not have the same type as the target element.
	 */
	public static final String DESIGN_EXCEPTION_WRONG_TYPE = MessageConstants.EXTENDS_EXCEPTION_WRONG_TYPE;
	/**
	 * The parent element does not have the same type of extension as the target
	 * element. Both the parent and the target element are ExtendedItem.
	 */
	public static final String DESIGN_EXCEPTION_WRONG_EXTENSION_TYPE = MessageConstants.EXTENDS_EXCEPTION_WRONG_EXTENSION_TYPE;
	/**
	 * The element does not allow to set extends explicitly.
	 */
	public static final String DESIGN_EXCEPTION_EXTENDS_FORBIDDEN = MessageConstants.EXTENDS_EXCEPTION_EXTENDS_FORBIDDEN;
	/**
	 * The element does not allow extensions.
	 */
	public static final String DESIGN_EXCEPTION_CANT_EXTEND = MessageConstants.EXTENDS_EXCEPTION_CANT_EXTEND;
	/**
	 * The element cannot extend from itself.
	 */
	public static final String DESIGN_EXCEPTION_SELF_EXTEND = MessageConstants.EXTENDS_EXCEPTION_SELF_EXTEND;
	/**
	 * The extension would create a cycle: a extends b extends a.
	 */
	public static final String DESIGN_EXCEPTION_CIRCULAR = MessageConstants.EXTENDS_EXCEPTION_CIRCULAR;
	/**
	 * The parent element has no name.
	 */
	public static final String DESIGN_EXCEPTION_UNNAMED_PARENT = MessageConstants.EXTENDS_EXCEPTION_UNNAMED_PARENT;
	/**