Rollup merge of #154075 - nnethercote:rewrite-query_ensure_result, r=Zalathar

Rewrite `query_ensure_result`.

It currently uses chaining which is concise but hard to read. There are up to four implicit matches occurring after the call to `execute_query_fn`.
- The first `map` on `Option<Erased<Result<T, ErrorGuaranteed>>>`.
- The second `map` on `Option<Result<T, ErrorGuaranteed>>`.
- The third `map` on `Result<T, ErrorGuaranteed>`.
- The `unwrap_or` on `Option<Result<(), ErrorGuaranteed>>`.

This commit rewrites it to use at most two matches.
- An explicit match on `Option<Erased<Result<T, ErrorGuaranteed>>>`.
- An explicit match on `Result<T, ErrorGuaranteed>`.

This is easier to read. It's also more efficient, though the code isn't hot enough for that to matter.

r? @Zalathar