Issue #20508: Improve exception message of IPv{4,6}Network.__getitem__

Patch by Gareth Rees.
diff --git a/Lib/ipaddress.py b/Lib/ipaddress.py
index 1f90058..20f33cb 100644
--- a/Lib/ipaddress.py
+++ b/Lib/ipaddress.py
@@ -636,12 +636,12 @@
         broadcast = int(self.broadcast_address)
         if n >= 0:
             if network + n > broadcast:
-                raise IndexError
+                raise IndexError('address out of range')
             return self._address_class(network + n)
         else:
             n += 1
             if broadcast + n < network:
-                raise IndexError
+                raise IndexError('address out of range')
             return self._address_class(broadcast + n)
 
     def __lt__(self, other):