Improve error message of //tools/licenses.py.

tools/licenses.py

 #!/usr/bin/env python
 # Copyright (c) 2012 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.
 
 """Utility for checking and processing licensing information in third_party
 directories.
 
 Usage: licenses.py <command>
 
@@ skipping 340 matching lines @@
     if optional_keys is None:
         optional_keys = []
 
     if path in SPECIAL_CASES:
         metadata.update(SPECIAL_CASES[path])
     else:
         # Try to find README.chromium.
         readme_path = os.path.join(root, path, 'README.chromium')
         if not os.path.exists(readme_path):
             raise LicenseError("missing README.chromium or licenses.py "
-                               "SPECIAL_CASES entry")
+                               "SPECIAL_CASES entry in %s" % path)
 
         for line in open(readme_path):
             line = line.strip()
             if not line:
                 break
             for key in metadata.keys() + optional_keys:
                 field = key + ": "
                 if line.startswith(field):
                     metadata[key] = line[len(field):]
 
@@ skipping 259 matching lines @@
                                args.output_file, args.target_os,
                                args.gn_out_dir, args.gn_target, args.depfile):
             return 1
     else:
         print __doc__
         return 1
 
 
 if __name__ == '__main__':
   sys.exit(main())