Fix gyp-explain.py if dump.json doesn't exist.

tools/gyp-explain.py

 #!/usr/bin/env python
 # Copyright (c) 2011 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.
 
 """Prints paths between gyp targets.
 """
 
 import json
 import os
@@ skipping 42 matching lines @@
 
 def Main(argv):
   # Check that dump.json exists and that it's not too old.
   dump_json_dirty = False
   try:
     st = os.stat('dump.json')
     file_age_s = time.time() - st.st_mtime
     if file_age_s > 2 * 60 * 60:
       print 'dump.json is more than 2 hours old.'
       dump_json_dirty = True
-  except IOError:
+  except OSError:
     print 'dump.json not found.'
     dump_json_dirty = True
 
   if dump_json_dirty:
     print 'Run'
     print '    GYP_GENERATORS=dump_dependency_json build/gyp_chromium'
     print 'first, then try again.'
     sys.exit(1)
 
   g = json.load(open('dump.json'))
 
   if len(argv) != 3:
     usage()
     sys.exit(1)
 
   fro = MatchNode(g, argv[1])
   to = MatchNode(g, argv[2])
 
   paths = list(GetPath(g, fro, to))
   if len(paths) > 0:
     print 'These paths lead from %s to %s:' % (fro, to)
     for path in paths:
       print ' -> '.join(path)
   else:
     print 'No paths found from %s to %s.' % (fro, to)
 
 
 if __name__ == '__main__':
   Main(sys.argv)