Remove all uses of `todo!`

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

We have a number of uses of `todo!` that we should replace with use of `Err`.

```
$ find . -name "*.rs" -exec grep -H "todo\!" {} \;
./src/expr/indexed_field.rs: _ => todo!(),
./src/expr.rs: ScalarValue::Null => todo!(),
./src/expr.rs: ScalarValue::Decimal256(_, _, _) => todo!(),
./src/expr.rs: ScalarValue::FixedSizeBinary(_, _) => todo!(),
./src/expr.rs: ScalarValue::List(_) => todo!(),
./src/expr.rs: ScalarValue::Struct(_, _) => todo!(),
./src/expr.rs: ScalarValue::Dictionary(_, _) => todo!(),
./src/expr.rs: ScalarValue::FixedSizeList(_) => todo!(),
./src/expr.rs: ScalarValue::LargeList(_) => todo!(),
./src/expr.rs: Operator::AtArrow | Operator::ArrowAt => todo!(),
./src/common/data_type.rs: todo!()
./src/common/data_type.rs: ScalarValue::LargeList(_) => todo!(),
```

**Describe the solution you'd like**

**Describe alternatives you've considered**

**Additional context**

Start with the `todo!` matches in `src/expr/indexed_field.rs`, `src/expr.rs`, and `src/common/data_type.rs`, then trace the surrounding result and error handling. Check how these expression and data type paths represent unsupported cases and identify the relevant tests before replacing each placeholder. Done means the listed Rust sources contain no `todo!` uses and the existing test suite passes.

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