The reference "javax.faces.error.xhtml" isn't an error page but an identifier that ui:include recognizes and takes special action upon.
If your application defines an error-page in the web.xml, the default Facelets error page will not be displayed, however, if you use <ui:include src="javax.faces.error.xhtml"/> you can embed a similar view of the content generated by Facelets within your error page.
Note that the include will only work when ProjectStage is Development.
And lastly, yes, this feature is implemented (post-EDR2, so you'll have to use a nightly).
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