Improve temporary storage management.

Print the error message on local GAE instance failure.

Useful to debug when a new version of the GAE SDK breaks something.

TBR=iannucci@chromium.org
BUG=

Review URL: https://chromiumcodereview.appspot.com/13997004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@194131 0039d316-1c4b-4281-b951-d872f2087c98