Make the error message for unsupported operand types cleaner, in

response to a message by Laura Creighton on c.l.py.  E.g.

    >>> 0+''
    TypeError: unsupported operand types for +: 'int' and 'str'

(previously this did not mention the operand types)

    >>> ''+0
    TypeError: cannot concatenate 'str' and 'int' objects

Objects/stringobject.c

@@ -691,7 +691,7 @@ string_concat(register PyStringObject *a, register PyObject *bb)
 		    return PyUnicode_Concat((PyObject *)a, bb);
 #endif
 		PyErr_Format(PyExc_TypeError,
-			     "cannot add type \"%.200s\" to string",
+			     "cannot concatenate 'str' and '%.200s' objects",
 			     bb->ob_type->tp_name);
 		return NULL;
 	}