cros flash: log warnings when devserver log does not exist
cros flash: log warnings when devserver log does not exist

The non-existence of devserver log is the not cause of
failure. Instead of logging it as an error, we should log it as a
warning so that users don't get confused.

BUG=chromium:365870
TEST=None

Change-Id: I5d75c1c4bc1a937c8b2ce84c0a5badcdab9db395
Reviewed-on: https://chromium-review.googlesource.com/196527
Tested-by: Yu-Ju Hong <52dc720c411f8e2156e608c1a8b894a917489667@chromium.org>
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Commit-Queue: 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