SafeHtmlTemplate display alternative (backgorund- or cell-) image inside (mgwt)CellList-Item if image not available

Today I am rendering my mgwt CellList with the following Template and (rendering) method:

public interface Template extends SafeHtmlTemplates{
 @SafeHtmlTemplates.Template("<div style='{0}' class='{2}'>'{1}'</div>")
 SafeHtml content(SafeStyles dynamicBackgroundImage, String cellContents,
 String cssMyCell);
 }

public void render(SafeHtmlBuilder safeHtmlBuilder, ActivityItem model) {
 ArrayList<String> images = model.getImages();
 
 SafeStylesBuilder safeStylesBuilder = new SafeStylesBuilder();
 safeStylesBuilder.trustedBackgroundImage("url('" + images.get(0) + "')");
 
 String cssMyCell = SingletonClientResources.getInstance().getCssResources().getActivityItemCellCss().activityItemCell();
 
 SafeHtml content = TEMPLATE.content(safeStylesBuilder.toSafeStyles(),model.getTitle(), cssMyCell);
 safeHtmlBuilder.append(content);
 }

Sometimes the Image URL #1 (images.get(0)) is not valid/available and there should appear a default Image. So Chrome dev console tells me e.g: 

GET http://xyz.jpg 403 (Access Denied)

If this occurs, I want to display an alternative Image e.g. from my ClientBundle.

How can I handle this in an elegant way? 

(I know I could test all URLs by adding an Image Widget and listening for ErrorHandler. Also I don't want to check the URLs on Server side because the server is not in my influence)

THX

Marian

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