[Chromedriver] Return spec-compliant error in AddCookie command.

Bug: chromedriver:1887, chromedriver:2002
Change-Id: I327ba44051de581075b6c14132b63d1422556847
Reviewed-on: https://chromium-review.googlesource.com/578609
Commit-Queue: Gauri Manikpure <gmanikpure@chromium.org>
Reviewed-by: Shuotao Gao <stgao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502721}
4 files changed