Correct error message for matrix length

[radarhere]

Suggested in #9726 (comment)

Before

>>> from PIL import Image
>>> im = Image.new("RGB", (1, 1))
>>> im.convert("L", matrix=(0,))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "PIL/Image.py", line 1088, in convert
    im = self.im.convert_matrix(mode, matrix)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: argument 2 must be sequence of length 12, not 1

However, matrix can actually have length 4 or 12

After

>>> from PIL import Image
>>> im = Image.new("RGB", (1, 1))
>>> im.convert("L", matrix=(0,))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "PIL/Image.py", line 1088, in convert
    im = self.im.convert_matrix(mode, matrix)
         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: matrix must be tuple of length 4 or 12

[akx]

That's nice!