Make `download_material_design_icons` work in a clean checkout.

sky/sdk/lib/download_material_design_icons

 #!/usr/bin/env python
 # Copyright (c) 2015 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import os
 import shutil
 import subprocess
 import sys
 import urllib2
 
+
 def main():
     sky_lib_dir = os.path.dirname(os.path.abspath(__file__))
     assets_dir = os.path.join(sky_lib_dir, 'assets')
     icons_dir = os.path.join(assets_dir, 'material-design-icons')
     existing_sha1_path = os.path.join(icons_dir, 'material-design-icons.sha1')
 
     existing_sha1 = None
     if os.path.isfile(existing_sha1_path):
         with open(existing_sha1_path, 'r') as f:
             existing_sha1 = f.read()
 
     sha1_path = os.path.join(assets_dir, 'material-design-icons.sha1')
 
     with open(sha1_path, 'r') as f:
         sha1 = f.read()
 
     if existing_sha1 == sha1:
         return
 
     print "Downloading missing material design icons"
 
     tgz_path = os.path.join(assets_dir, 'material-design-icons.tgz')
     url = 'https://storage.googleapis.com/mojo/material-design-icons/%s' % sha1
     response = urllib2.urlopen(url)
 
     with open(tgz_path, 'wb') as f:
         f.write(response.read())
 
-    shutil.rmtree(icons_dir)
+    shutil.rmtree(icons_dir, ignore_errors=True)
 
     output_path = os.path.join(assets_dir, tgz_path)
     subprocess.call([
         'tar', '-xzf', output_path, '-C', assets_dir
     ])
 
     subprocess.call([
         'cp', sha1_path, icons_dir
     ])
 
     os.unlink(tgz_path)
 
 if __name__ == '__main__':
     sys.exit(main())