feat(python): improved exceptions on attempt to use invalid schema/dtypes

[alexander-beedie]

Closes #6648.

Enables centralised dtype sanity checking for schema params inside _unpack_schema, and raises more helpful errors in general for attempts to use invalid dtypes.

---

Examples:

• Use of invalid dtype in schema.

  pl.DataFrame(
      data = {"words": [["hello", "hi"], ["polar", "bears"]]}, 
      schema = {"words": pl.list(pl.Categorical)},
  )

  Before

  ValueError: Since Expr are lazy, the truthiness of an Expr is ambiguous. Hint: use '&' or '|' to logically combine Expr, not 'and'/'or', and use 'x.is_in([y,z])' instead of 'x in [y,z]' to check membership.
  

  After

  ValueError: Cannot infer dtype from 'COLUMN OF DTYPE: [Categorical(None)].list()' (type: Expr)
  

• Use of invalid dtype in cast operation.

  df1 = pl.DataFrame({"words": [["hello", "hi"], ["polar", "bears"]]})
  df1.select(pl.col("words").cast(pl.list(pl.Categorical)))

  Before

  ValueError: could not convert value 'Unknown' as a Literal
  

  After

  ValueError: Cannot infer dtype from 'COLUMN OF DTYPE: [Categorical(None)].list()' (type: Expr)