cros flash: output error message when failed to start devserver

BUG=chromium:353040
TEST=Ran `cros flash` with cherrypy uninstalled.

Change-Id: Iaad18a43845480477958212ea3e92474f1b9eda2
Reviewed-on: https://chromium-review.googlesource.com/190342
Reviewed-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
Commit-Queue: Yu-Ju Hong <yjhong@chromium.org>
Tested-by: Yu-Ju Hong <yjhong@chromium.org>
1 file changed