Another solution is to override the template for the default actionError output.
You can find the default templates in the struts2 core jar. If you pull out template.simple/actionerror.ftl, you can customize that. You can either come up with your own template and reference it in the s:actionerror tag with the template attribute, or you can keep the same name and put it in /template/simple and it will be used as the default.
Most of the templates are in freemarker, although there are still some of them in velocity depending on your struts2 version. Both are pretty easy to work with templating engines.