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**