how to share a jsf error page between multiple wars

You need to create a custom ResourceResolver which resolves resources from classpath, put it in the common JAR file and then declare it in web-fragment.xml of the JAR (or in web.xml of the WARs).

Kickoff example:

package com.example;

import java.net.URL;

import javax.faces.view.facelets.ResourceResolver;

public class FaceletsResourceResolver extends ResourceResolver {

    private ResourceResolver parent;
    private String basePath;

    public FaceletsResourceResolver(ResourceResolver parent) {
        this.parent = parent;
        this.basePath = "/META-INF/resources"; // TODO: Make configureable?
    }

    @Override
    public URL resolveUrl(String path) {
        URL url = parent.resolveUrl(path); // Resolves from WAR.

        if (url == null) {
            url = getClass().getResource(basePath + path); // Resolves from JAR.
        }

        return url;
    }

}

with in web-fragment.xml or web.xml

<context-param>
    <param-name>javax.faces.FACELETS_RESOURCE_RESOLVER</param-name>
    <param-value>com.example.FaceletsResourceResolver</param-value>
</context-param>