This changes the traceback from

  Traceback (most recent call last):
    File ".../matplotlib/__init__.py", line 768, in __setitem__
      cval = self.validate[key](val)
    File ".../matplotlib/rcsetup.py", line 67, in __call__
      raise ValueError('Unrecognized %s string %r: valid strings are %s'
  ValueError: Unrecognized capstyle string 'foobar': valid strings are ['butt', 'round', 'projecting']

  During handling of the above exception, another exception occurred:

  Traceback (most recent call last):
    File "<string>", line 4, in <module>
    File ".../matplotlib/__init__.py", line 770, in __setitem__
      raise ValueError("Key %s: %s" % (key, str(ve)))
  ValueError: Key lines.solid_capstyle: Unrecognized capstyle string 'foobar': valid strings are ['butt', 'round', 'projecting']

to

  Traceback (most recent call last):
    File "<string>", line 4, in <module>
    File ".../matplotlib/__init__.py", line 770, in __setitem__
      raise ValueError(f"Key {key}: {ve}") from None
  ValueError: Key lines.solid_capstyle: Unrecognized capstyle string 'foobar': valid strings are ['butt', 'round', 'projecting']