Add missing non_exhaustive enum variant constructors to SlashArgError #962

Job	Run time
Clippy	48s
Rustfmt	14s
Build docs	1m 23s
Test (MSRV)	1m 25s
Test (stable)	2m 0s
Test (beta)	2m 10s
Test (nightly)	1m 59s
Test (macOS)	6m 3s
Test (Windows)	4m 2s
Test (no features)	1m 13s
Test (all features - simdjson, cargo test --all-features --lib --tests --examples)	1m 18s
Test (native TLS, cargo test --all-features --features serenity/native_tls_backend --lib --tests ...	1m 21s
	23m 56s

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