Print a useful message when test scripts cannot update a running dartium.

tools/get_archive.py

 #!/usr/bin/env python
 #
 # Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 # Gets or updates a content shell (a nearly headless build of chrome). This is
 # used for running browser tests of client applications.
 
 import json
@@ skipping 270 matching lines @@
                         (temp_zip, temp_dir, str(out)))
       unzipped_dir = temp_dir + '/' + os.path.basename(latest)[:-len('.zip')]
     else:
       z = zipfile.ZipFile(temp_zip)
       z.extractall(temp_dir)
       unzipped_dir = os.path.join(temp_dir,
                                   os.path.basename(latest)[:-len('.zip')])
       z.close()
     if directory == SDK_DIR:
       unzipped_dir = os.path.join(temp_dir, 'dart-sdk')
+    if os.path.exists(directory):
+      raise Exception(
+          'Removal of directory %s failed. Is the executable running?' %
+          directory)
     shutil.move(unzipped_dir, directory)
   finally:
     shutil.rmtree(temp_dir)
 
   # create the version stamp
   v = open(version_file, 'w')
   v.write(latest)
   v.close()
 
   print 'Successfully downloaded to %s' % directory
@@ skipping 46 matching lines @@
     GetDartiumRevision('content_shell', bot, DRT_DIR, DRT_VERSION,
                          DRT_LATEST_PATTERN, DRT_PERMANENT_PATTERN,
                          args.revision)
     CopyDrtFont(DRT_DIR)
   else:
     print ('Please specify the target you wish to download from Google Storage '
         '("drt", "dartium", "chromedriver", or "sdk")')
 
 if __name__ == '__main__':
   sys.exit(main())