nebraska: Send responses in binary format

Seems like sending error responses in `str` fails. Sending in binary
format is requested.

BUG=None
TEST=unittests

Change-Id: I50f7d4802745b57c30120847195084f9e3277adc
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/2952948
Tested-by: Amin Hassani <ahassani@chromium.org>
Reviewed-by: Vyshu Khota <vyshu@chromium.org>
Reviewed-by: Jae Hoon Kim <kimjae@chromium.org>
Commit-Queue: Jae Hoon Kim <kimjae@chromium.org>
2 files changed