Document error 20 in SDK, improve errors

[maxim-inj]

Quoting some concern:

For a smoother developer experience, it makes sense to document error 20 right in our SDK—exactly where users run into it. This keeps our SDK a true one‑stop resource: engineers get answers in seconds instead of detouring through Cosmos‑SDK manuals or general docs, and we can still link to the upstream docs for extra detail later. It’s a small addition that removes friction and underscores our commitment to developer‑first tooling.

Also, the documentation has errors specified per module, which actually clutters the search a lot. Ideally we want one registry page with all errors listed and explained. Having errors per-module is useful for internal devs, while nearly all users are getting these from a single endpoint or sdk call.