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
* 
	 * @param obj
	 *            the element being changed.
	 * @param parent
	 *            the parent element.
	 * @param errCode
	 *            what went wrong.
	 */
	public ExtendsException( DesignElement obj, DesignElement parent,
			String errCode )
	{
		super( obj, errCode );
		this.parent = parent;
		this.extendsName = parent.getName( );
	}
	/**
	 * Returns the attempted new value for the extends property.
	 * 
	 * @return the parent element name.
	 */
	public Object getExtends( )
	{
		return extendsName;
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see java.lang.Throwable#getLocalizedMessage()
	 */
	public String getLocalizedMessage( )