hg: build-infra/jdk8: Properly detect when python cannot be found from hg script.

[fredrik.ohrstrom at oracle.com]

Changeset: fe40f0641f3c
Author:    ohrstrom
Date:      2012-11-29 14:48 +0100
URL:       http://hg.openjdk.java.net/build-infra/jdk8/rev/fe40f0641f3c

Properly detect when python cannot be found from hg script.

! common/bin/hgforest.sh