RFR 8189671: jlink should clearly report error when an automatic module is used as root

[Sundararajan Athijegannathan]

Please review.

Bug: https://bugs.openjdk.java.net/browse/JDK-8189671
Webrev: http://cr.openjdk.java.net/~sundar/8189671/webrev.00/

Thanks,
-Sundar