Make setup_links.py not fail if Chromium checkout is missing.

If a checkout has been created but haven't yet executed gclient
runhooks, running setup_links.py --clean-only will fail if
the Chromium checkout isn't yet synced. This can make bots
end up in a bad state since we now clean all links before running
bot_update. Relaxing this error solves that problem.

BUG=chromium:663278
TBR=ehmaldonado@webrtc.org
NOTRY=True

Review URL: https://codereview.webrtc.org/2496323004 .

Cr-Commit-Position: refs/heads/master@{#15097}

setup_links.py

@@ -507,9 +507,10 @@ def main():
                       'administrator. Please run with user account privileges.')
 
   if not os.path.exists(CHROMIUM_CHECKOUT):
-    logging.error('Cannot find a Chromium checkout at %s. Did you run "gclient '
-                  'sync" before running this script?', CHROMIUM_CHECKOUT)
-    return 2
+    logging.warning('Cannot find a Chromium checkout at %s. Did you run '
+                    '"gclient sync" before running this script?',
+                    CHROMIUM_CHECKOUT)
+    return 0
 
   links_database = _initialize_database(LINKS_DB)
   try: