faft: Better error messages on firmware_ECBattery test

Print the detailed values when failure.

BUG=chrome-os-partner:14157
TEST=Run firmware_ECBattery passed on Snow.

Change-Id: I62558ccacc0f0c3975fc39fdbdb64ee14f002b16
Reviewed-on: https://gerrit.chromium.org/gerrit/33482
Reviewed-by: Mike Truty <truty@chromium.org>
Commit-Ready: Tom Wai-Hong Tam <waihong@chromium.org>
Tested-by: Tom Wai-Hong Tam <waihong@chromium.org>
1 file changed