Fix incorrect use of errors.Append masking errors
Created by: jhchabran
Follow-up to https://github.com/sourcegraph/sourcegraph/pull/34563
It's possible to pass only a single argument to errors.Append which has the effect of not appending the errors and only returning the last one.
It's really easy to miss because the API makes it look like you're calling an Append method on a receiver, which would imply it has a side-effect. But it's not a receiver it's a package, so it just silently discards errors.
Test plan
Minor change, green build.