Fix confusing error message when cherrypy is not installed on DUT.

Also fixed a few warnings.

BUG=None
TEST=Installed a couple of images on machines with and without cherrypy.

Change-Id: I40d3fa1882e3bceb0e70a8d040a892e894cf50db
Reviewed-on: https://chrome-internal-review.googlesource.com/217045
Reviewed-by: Caroline Tice <cmtice@google.com>
Tested-by: Luis Lozano <llozano@chromium.org>
Commit-Queue: Luis Lozano <llozano@chromium.org>
2 files changed