This reverts commit 2c199e1e.

Reason for revert: This makes calls to gclient that have the only intention of updating gclient fail, like:
https://cs.chromium.org/chromium/src/v8/tools/try_perf.py?l=93

Reverting for now to give time to clean up such scripts before reland.

Original change's description:
> gclient: return non-zero exit code on unknown command
> 
> Bug: none
> Change-Id: I447f66765679b7b66b5748af1cf1f501610603bf
> Reviewed-on: https://chromium-review.googlesource.com/504408
> Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
> Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
> 

TBR=iannucci@chromium.org,phajdan.jr@chromium.org,dpranke@chromium.org,tandrii@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true
Bug: none

Change-Id: I9496f7192dfde1e38c186a94ac985190b76b2438
Reviewed-on: https://chromium-review.googlesource.com/506563Reviewed-by: Michael Achenbach <machenbach@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>