chore: Fix error checking in codeintel source
Created by: efritz
This updates two shared error helper methods:
- vcs.IsRepoNotExist
- gitserver.IsRevisionNotFound
Both methods now unwrap their arguments to check for type equality.
The remaining changes are updates to codeintel usage of error checking. See https://github.com/sourcegraph/sourcegraph/pull/22657 for context on a previous attempt.