hg: shenandoah/jdk9/hotspot: Exit gracefully when Shenandoah is not supported.

[ashipile at redhat.com]

Changeset: 047a2d61088f
Author:    shade
Date:      2017-03-21 12:12 +0100
URL:       http://hg.openjdk.java.net/shenandoah/jdk9/hotspot/rev/047a2d61088f

Exit gracefully when Shenandoah is not supported.

! src/share/vm/runtime/arguments.cpp