Make the error message for unsupported operand types cleaner, in

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