Make `download_material_design_icons` work in a clean checkout.

sky/sdk/lib/download_material_design_icons

Index: sky/sdk/lib/download_material_design_icons
diff --git a/sky/sdk/lib/download_material_design_icons b/sky/sdk/lib/download_material_design_icons
index 02ecfba3be57f6a2e7a0c3db036797019959bf1e..f0d987e646b6a752d019e7405e9c118c7b7aa1a2 100755
--- a/sky/sdk/lib/download_material_design_icons
+++ b/sky/sdk/lib/download_material_design_icons
@@ -9,6 +9,7 @@ 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')
@@ -37,7 +38,7 @@ def main():
     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([