unpacker: check the right exception type for 2.4

mercurial/util.py

@@ -234,7 +234,7 b' def unpacker(fmt):'
     try:
         # 2.5+
         return struct.Struct(fmt).unpack
-    except NameError:
+    except AttributeError:
         # 2.4
         return lambda buf: struct.unpack(fmt)