Clean up Result types for WsClient

[mkrasnitski]

Follow-up to #3369 - using StdResult isn't necessary because we can already coerce tungstenite::Error into serenity's Error. Worth noting that this incurs a Box call for the error type, but this isn't a hot path by any means.

[GnomedDev]

This was a purposeful optimisation to avoid returning the large enum of serenity's Error and instead just return the smaller tungstenite error. Since these are internal methods, it doesn't matter to be hiding the error type, so I cannot see the benefit here.

If you have a benefit to reverting this, let me know, otherwise I would like to see this PR closed.

[mkrasnitski]

Turns out serenity::Error is pretty small, 72 bytes. And, tokio_tungstenite::tungstenite::Error is 136 bytes large. So, unless we box it, we're not getting any benefit. Plus, mixing result types like this is odd imo.

[GnomedDev]

Fair enough.