* @param problem The problem whose location should be formatted, must not be {@code null}.
     * @param projectId The {@code <groupId>:<artifactId>:<version>} of the corresponding project, may be {@code null}
     *            to force output of model id and source.
     * @return The formatted problem location or an empty string if unknown, never {@code null}.
     */
    public static String formatLocation(ModelProblem problem, String projectId) {