`SlashArgError` variants are private and have no constructor functions

[kb-1000]

I've been trying to implement SlashArgument for a custom Mentionable enum (containing Member, not User), but while SlashArgError::new_command_structure_mismatch exists, there is no way to construct SlashArgError::Invalid the way Member's implementation does.

It'd be nice to have such an enum in the framework as well, of course (and it does seem like I can just call Member::extract myself), but since it's not documented that you aren't supposed to implementSlashArgument yourself, this surprises me.

[inklesspen]

I ran into this issue myself today. Apparently back in #234 the Parse and Invalid variants were described as framework-internal. That's strange, because the SlashArgument trait is public and documented as something you might want to implement on types, but for every type on which you might want to implement it, it would make sense to return Parse or Invalid errors.