Bug 159076 PDF emitter needs to response if "com.lowagie.itext" is missed.
Previously on GUI, preview in PDF has to check the "com.lowagie.itext" is included in BIRT firstly. Now we think GUI doesn't need to do any check for emitters and in order to avoid any hard coded behaviors about emitter contribution, this check is remove from GUI codes. So, PDF emitter should be responsible to response if required resources is missed.
engine/org.eclipse.birt.report.engine/src/org/eclipse/birt/report/engine/api/impl/RenderTask.java
{
	IReportDocument reportDoc;
	String emitterID;
	private InnerRender innerRender;
	/**
	 * @param engine
	 *            the report engine
	 * @param runnable
	 *            the report runnable object
	 * @param reportDoc
	 *            the report document instance
	 */
	public RenderTask( IReportEngine engine, IReportRunnable runnable,
			IReportDocument reportDoc )
	{
		super( engine, runnable );
		executionContext.setFactoryMode( false );
		executionContext.setPresentationMode( true );
		
		openReportDocument( reportDoc );
		
		
		loadDesign( );
		innerRender = new PageRangeRender( new long[]{1,
				this.reportDoc.getPageCount( )} );
	}
	protected void openReportDocument( IReportDocument reportDoc )
	{