Prune the catch-all Error type in the client crate

[hdevalence]

The client crate has a giant catch-all Error type, many of whose variants are unnecessary now.

For each variant, we should check if it is ever constructed (eg using vs code) and delete it if not.

[conorsch]

Sounds like a fairly straightforward task. Keeping in Next, so we can perform this cleanup ahead of upcoming audits.