Add missing non_exhaustive enum variant constructors to SlashArgError

[kangalio]

Actually, looking at this code - there was a constructor function but it was #[doc(hidden)], because I didn't intend it to be used outside the lib. For example the Parse variant is intended for poise's FromStr adapter (i.e. if a FromStr type is used as a parameter and it fails, Parse is thrown).

Why do you need to construct these framework-internal (intended as such, at least) variants in user code?

[kangalio]

By the way, thank you! I sometimes forget to appreciate the effort of people submitting PRs

[webmsgr]

I was using a command argument with a FromStr impl, but because the error type didn't implement Error (anyhow), I got some very confusing errors. I tried to manually implement both PopArgument and SlashArgument (which some of the errors suggested) and ran into the issue I couldn't create a SlashArgError::Parse for when the FromStr fails inside the SlashArgument impl. I've basically scrapped the whole thing and just call FromStr manually now, so feel free to close if you want.

Two of the errors pictured below:

[kangalio]

The FromStr error type was anyhow::Error? That's really sus. What crate is that, so I can raise an issue?

[webmsgr]

Its part of the bots source code. Its probably not the best idea, but i was already using it and didn't want to over complicate things.