hg: shenandoah/jdk10: Improve assertion/verification messages a bit

[ashipile at redhat.com]

Changeset: cac659ba1f08
Author:    shade
Date:      2018-02-14 16:58 +0100
URL:       http://hg.openjdk.java.net/shenandoah/jdk10/rev/cac659ba1f08

Improve assertion/verification messages a bit

! src/hotspot/share/gc/shenandoah/shenandoahAsserts.cpp
! src/hotspot/share/gc/shenandoah/shenandoahVerifier.cpp