Better argument reader in contracts.

When reading more then one argument it is hard to guess which one is missing, because both throw `ApiError::MissingArgument`. It would be handy to have `get_named_arg` that returns `Result` or `Option`. This way I could manually throw custom errors like `AmountArgIsMissing`.

To keep the backward compatibility I'd suggest adding new method `try_get_named_arg` to `casper-contrac` crate.