Add required `error` method to `Decoder`
Add required `error` method to `Decoder`

Since rustc pull #16130 there has been a new method in the `Decoder`
trait for signalling application-specific decode errors.

badboy/toml-rs,vosen/toml_document,GuillaumeGomez/toml-rs,vosen/toml-rs,mseri/toml-rs,roguePanda/toml-rs,trlim/toml-rs,alexcrichton/toml-rs