Change APIs to return errors that implement `std::error::Error`

[LHolten]

Currently all APIs return Option<T> or Result<O, TableRow<T>> etc.
I think these APIs should return different error types that implement std::error::Error.
That will make it easier to quickly handle errors with ?.
I can include all relevant context to make these errors useful by default.

[LHolten]

Insert and update statements need a new error type for when there are multiple unique constraints.
The new error will allow extracting one of the rows that is conflicting and it will contain the column names that are involved in the conflict.