lib/errors: Simplify ClassifiedError type to Warning
Created by: indradhanush
Following up on post merge review received here.
If we are trying to expose two "levels" of errors, one being warning and another error, it is simpler to just expose a custom type Warning to replicate the warning level of errors, and all other errors are just errors.
Part of #38612. Fixes #38615 (closed).
Test plan
Added tests.