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
services.setRenderOption( renderOptions );
		services.setExecutor( executor );
		services.setRenderContext( appContext );
		services.setReportRunnable( runnable );
		
		emitter.initialize( services );
	}
	/*
	 * (non-Javadoc)
	 * 
	 * @see org.eclipse.birt.report.engine.api.IRenderTask#setEmitterID(java.lang.String)
	 */
	public void setEmitterID( String id )
	{
		this.emitterID = id;
	}
	/**
	 * @return the emitter ID to be used to render this report. Could be null,
	 *         in which case the engine will choose one emitter that matches the
	 *         requested output format.
	 */
	public String getEmitterID( )
	{
		return this.emitterID;
	}
	public void close( )
	{
		closeReportDocument( );
		super.close( );
	}