Add error types to declaration file

[milindl]

Adds error types to declaration. Re-exports error codes as needed. Also removes unused error.

[confluent-cla-assistant]

🎉 All Contributor License Agreements have been signed. Ready to merge.
_{Please push an empty commit if you would like to re-run the checks to verify CLA status for all contributors.}

[emasab]

Not related to this PR, but there's a linting error to fix in

src/admin.cc:1001:  Lines should be <= 80 characters long  [whitespace/line_length] [2]

In semaphore.yml could you change name: "ESLint" to name: "Lint" and run make lint instead?

For the rest it looks good.