chore(runner): return errors created by connect (go-gitea#7222)

- Instead of creating errors via `google.golang.org/grpc`, use `connectrpc.com/connect`.
- This _avoids_ another dependency (still indirectly referenced in testing).

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7222
Reviewed-by: Otto <[email protected]>
Co-authored-by: Gusted <[email protected]>
Co-committed-by: Gusted <[email protected]>

Author: Gusted