Log stdout on postinst fail.

BUG=chrome-os-partner:13103
TEST=Modify postinst invocation to simulate failure,
  ensure that postinst stdout is present in the test log

Change-Id: I0da39517e6a45f699000adee44bc0a7e7144cb4a
Reviewed-on: https://gerrit.chromium.org/gerrit/33681
Reviewed-by: Hung-Te Lin <hungte@chromium.org>
Commit-Ready: Jon Salz <jsalz@chromium.org>
Tested-by: Jon Salz <jsalz@chromium.org>
diff --git a/py/gooftool/gooftool.py b/py/gooftool/gooftool.py
index 4ff7aab..11f8873 100755
--- a/py/gooftool/gooftool.py
+++ b/py/gooftool/gooftool.py
@@ -525,7 +525,8 @@
   rootfs_device = GetReleaseRootPartitionPath()
   result = Shell('%s %s' % (script, rootfs_device))
   if not result.success:
-    raise Error, '%r failed, stderr: %r' % (script, result.stderr)
+    raise Error, '%r failed, stdout: %r, stderr: %r' % (
+        script, result.stdout, result.stderr)
 
 
 @Command('verify_switch_wp')