[Fuchsia] Better error message from SDK update script

tools/download_latest_dev_sdk.py

 #!/usr/bin/env python
 # Copyright 2016 The Dart project authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This script downloads the latest dev SDK from
 # http://gsdview.appspot.com/dart-archive/channels/dev/raw/latest/sdk/
 # into tools/sdks/$HOST_OS/. It is intended to be invoked from Jiri hooks in
 # a Fuchsia checkout.
 
@@ skipping 16 matching lines @@
 def host_os_for_sdk(host_os):
   if host_os.startswith('macos'):
     return 'mac'
   if host_os.startswith('win'):
     return 'windows'
   return host_os
 
 # Python's zipfile doesn't preserve file permissions during extraction, so we
 # have to do it manually.
 def extract_file(zf, info, extract_dir):
-  zf.extract( info.filename, path=extract_dir )
-  out_path = os.path.join(extract_dir, info.filename)
-  perm = info.external_attr >> 16L
-  os.chmod(out_path, perm)
+  try:
+    zf.extract(info.filename, path=extract_dir)
+    out_path = os.path.join(extract_dir, info.filename)
+    perm = info.external_attr >> 16L
+    os.chmod(out_path, perm)
+  except IOError as err:
+    if 'dart-sdk/bin/dart' in err.filename:
+      print('Failed to extract the new Dart SDK dart binary. ' +
+            'Kill stale instances (like the analyzer) and try the update again')
+      return False
+    raise
+  return True
 
 def main(argv):
   host_os = host_os_for_sdk(HOST_OS)
   zip_file = ('dartsdk-%s-x64-release.zip' % HOST_OS)
   sha_file = zip_file + '.sha256sum'
   sdk_path = os.path.join(DART_ROOT, 'tools', 'sdks', host_os)
   local_sha_path = os.path.join(sdk_path, sha_file)
   remote_sha_path = os.path.join(sdk_path, sha_file + '.remote')
   zip_path = os.path.join(sdk_path, zip_file)
 
@@ skipping 18 matching lines @@
   urllib.urlretrieve(sha_url, remote_sha_path)
   with open(remote_sha_path, 'r') as fp:
     remote_sha = fp.read()
   os.remove(remote_sha_path)
 
   if local_sha == '' or local_sha != remote_sha:
     print 'Downloading prebuilt Dart SDK from: ' + zip_url
     urllib.urlretrieve(zip_url, zip_path)
     with zipfile.ZipFile(zip_path, 'r') as zf:
       for info in zf.infolist():
-        extract_file(zf, info, sdk_path)
+        if not extract_file(zf, info, sdk_path):
+          return -1
     with open(local_sha_path, 'w') as fp:
       fp.write(remote_sha)
+  return 0
 
 if __name__ == '__main__':
   sys.exit(main(sys.argv))