Fix gRPC error handling on Push.

Fix gRPC error handling on Push.

Tested: generated an ALREADY_EXISTS exception on the proxy and verified that
it's correctly ignored.

BUG=

Review-Url: https://codereview.chromium.org/2968343002
Committed: https://github.com/luci/luci-py/commit/9b6ac24b8798e83d363b6e492652bbeecf280128

[aludwin, 2017-07-07 04:17:22]

aludwin@google.com changed reviewers:
+ ryanmartens@google.com, vadimsh@chromium.org

[Vadim Sh., 2017-07-07 17:55:13]

lgtm

[aludwin, 2017-07-07 18:11:50]

The CQ bit was checked by aludwin@google.com

[commit-bot: I haz the power, 2017-07-07 18:12:02]

CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-07-07 18:45:34]

CQ is committing da patch.

Bot data: {"patchset_id": 1, "attempt_start_ts": 1499451110800210, "parent_rev":
"bd3cbc5ca8f6fd82345bb9073364c56bf2b73130", "commit_rev":
"9b6ac24b8798e83d363b6e492652bbeecf280128"}

[commit-bot: I haz the power, 2017-07-07 18:45:37]

Description was changed from

==========
Fix gRPC error handling on Push.

Tested: generated an ALREADY_EXISTS exception on the proxy and verified that
it's correctly ignored.

BUG=
==========

to

==========
Fix gRPC error handling on Push.

Tested: generated an ALREADY_EXISTS exception on the proxy and verified that
it's correctly ignored.

BUG=

Review-Url: https://codereview.chromium.org/2968343002
Committed:
https://github.com/luci/luci-py/commit/9b6ac24b8798e83d363b6e492652bbeecf280128
==========

[commit-bot: I haz the power, 2017-07-07 18:45:38]

Committed patchset #1 (id:1) as
https://github.com/luci/luci-py/commit/9b6ac24b8798e83d363b6e492652bbeecf280128