feat(rust,python): better-characterise NotFound exceptions

[alexander-beedie]

Closes #6647.

Rather than a single NotFound exception, we can now characterise this error by what was not found:

• ColumnNotFound
• SchemaFieldNotFound
• StructFieldNotFound

Example

df = pl.struct(
    [pl.Series("a", [1,2,3]), pl.Series("b", ["a","b","c"])], eager=True
).to_frame().with_columns( pl.lit(0).alias("b") )

Before (exception lacks context)

df.select(pl.col("c").struct.field("c"))   << which "c" causes the error?
# NotFoundError: c

df.select(pl.col("a").struct.field("c"))
# NotFoundError: c

After (exception is specific)

df.select(pl.col("c").struct.field("c"))
# ColumnNotFoundError: c

df.select(pl.col("a").struct.field("c"))
# StructFieldNotFoundError: c

[ritchie46]

Nice addition!