cros flash: output error message when failed to start devserver
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 <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Reviewed-by: Don Garrett <546ec366615570e68497e3c5df9092106a9808f4@chromium.org>
Commit-Queue: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Tested-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>

bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite,chadversary/chromiumos.chromite,bpsinc-native/src_third_party_chromite