branch_api: silently proceed when branch already exists

Previously we'd only proceed here if --force were enabled. However,
Cindy got hit with an error here even when it was her first attempt at
branching. I suspect the request to Gerrit might've been retried
somehow.

Anyway, this doesn't actually change the program's behavior in a
meaningful way, since the program fails prior to this error, at the
following block, if manifest-internal has already been branched.
http://cs/chromeos_public/infra/go/cmd/branch_util/create_with_apis.go?l=209-212&rcl=ab821cece8b984e46fb56a246c90c971614071b1

BUG=chromium:1069862
TEST=tests pass

Change-Id: Ib9b5802ad9a386dc46178a394b34e25f1fb288e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/infra/go/+/2276573
Reviewed-by: Navil Perez <navil@google.com>
Commit-Queue: Sean Abraham <seanabraham@chromium.org>
Tested-by: Sean Abraham <seanabraham@chromium.org>
3 files changed