Pg arguments add error type

[csandven]

fixes #3563

[abonander]

This is actually intentional because it's more complex if you actually want to handle the error in-place. You have to match on the sqlx::Error despite knowing that it's going to be Error::Encode.

What we should probably do here is return a new error type, e.g. EncodeError and have sqlx::Error implement From<EncodeError> so that you can use the ? operator if you want.