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

BUG=none
TEST=Run with no dump.json around.

Review URL: http://codereview.chromium.org/8770056

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@112752 0039d316-1c4b-4281-b951-d872f2087c98

tools/gyp-explain.py

@@ -60,7 +60,7 @@ def Main(argv):
     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