Fix error message when using exclusion

Patch by Sylvain Defresne <sdefresne@chromium.org>

Review URL: https://codereview.chromium.org/64633003/


git-svn-id: http://gyp.googlecode.com/svn/trunk@1786 78cadc50-ecff-11dd-a971-7dbc132099af
diff --git a/pylib/gyp/input.py b/pylib/gyp/input.py
index 45e791d..9bc449d 100644
--- a/pylib/gyp/input.py
+++ b/pylib/gyp/input.py
@@ -2278,6 +2278,7 @@
       continue
 
     if not isinstance(the_dict[list_key], list):
+      value = the_dict[list_key]
       raise ValueError, name + ' key ' + list_key + \
                         ' must be list, not ' + \
                         value.__class__.__name__ + ' when applying ' + \