Add useful error message when trying to add Slider to 3DAxes

[dstansby]

Before the error message would have been:

Traceback (most recent call last):
  File "test.py", line 6, in <module>
    valk = Slider(ax, 'kparam', 0, 30.0, valinit=1.0)
  File "/Users/dstansby/github/matplotlib/lib/matplotlib/widgets.py", line 381, in __init__
    horizontalalignment='right')
TypeError: text() missing 1 required positional argument: 's'

Now it is

Traceback (most recent call last):
  File "test.py", line 6, in <module>
    valk = Slider(ax, 'kparam', 0, 30.0, valinit=1.0)
  File "/Users/dstansby/github/matplotlib/lib/matplotlib/widgets.py", line 324, in __init__
    raise ValueError('Sliders cannot be added to 3D Axes')
ValueError: Sliders cannot be added to 3D Axes

I was wondering if this should be a more strict test for ax.name != 'rectilinear' instead, but sliders with at least a polar projection do seem to work (modulo some drawing issues).

Closes #15156