Add LogDog log stream fetcher code.

web/web.py

 #!/usr/bin/env python
 # Copyright 2016 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 """Manages web/ resource checkout and building.
 
 This script can be run in one of three modes:
   - As "initialize.py", it will perform resource dependency checkout for
     "luci_deploy" and quit.
@@ skipping 102 matching lines @@
   @property
   def apps_dir(self):
     return os.path.join(self._web_dir, 'apps')
 
   def _call(self, *args, **kwargs):
     LOGGER.debug('Running command (cwd=%s): %s',
         kwargs.get('cwd', os.getcwd()),
         pipes.quote(' '.join(args)))
 
     kwargs['stderr'] = subprocess.STDOUT
-    subprocess.check_call(args, **kwargs)
+    try:
+      subprocess.check_call(args, **kwargs)
+    except subprocess.CalledProcessError as e:
+      LOGGER.warning('Non-zero return code (%d) from command.',
+          e.returncode, exc_info=LOGGER.isEnabledFor(logging.DEBUG))
+      sys.exit(e.returncode)
 
   def node(self, *args, **kwargs):
     self._call(self._node_exe, *args, **kwargs)
 
   def npm(self, *args, **kwargs):
     self._call(self._npm_exe, *args, **kwargs)
 
   def bower(self, *args, **kwargs):
     exe = os.path.join(self.web_dir, 'node_modules', 'bower', 'bin', 'bower')
     return self.node(exe, *args, **kwargs)
@@ skipping 134 matching lines @@
     return _main_initialize(args)
   elif script_name == 'build.py':
     return _main_deploy(args)
   else:
     return _main(args)
 
 
 if __name__ == '__main__':
   logging.basicConfig(level=logging.INFO)
   sys.exit(main(sys.argv))