The attached patch (against trunk) changes the message.

However, it has at least one unintended consequence. If you have an
object with no __format__, it gets converted to a string, which is then
formatted. So you get:

>>> '{0:^10}'.format(0j)
'    0j    '
>>> '{0:^10x}'.format(0j)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: Unknown presentation type x for str
>>> 

I'm calling format on an complex number, but the error says "str". The
error is correct, because the complex number has been converted to a
string before the formatting mechanism can get the actual type. I think
we'll have to live with this if we add the type to the error message.