fix script so that the error is actually printed if ant is not installed

R=scroggo@google.com, hcm@google.com

Author: djsollen@google.com

Review URL: https://codereview.chromium.org/148793003

git-svn-id: http://skia.googlecode.com/svn/trunk@13290 2bbb7eff-a529-9590-31e7-b0007b416f81
1 file changed