Only print "step returned non-zero exit code" if return code is non-zero.
Only print "step returned non-zero exit code" if return code is non-zero.

R=agable@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/21845006

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@215645 0039d316-1c4b-4281-b951-d872f2087c98

shishkander/recipes-py,shishkander/recipes-py,luci/recipes-py,luci/recipes-py