Remove custom MultiError implementation and adopt hashicorp/go-multierror

The following discussion from !525 (merged) should be addressed:

• @jdrpereira started a discussion: (+1 comment)

  Using https://pkg.go.dev/github.com/hashicorp/go-multierror here to avoid reinventing the wheel. We built something much simpler in http://gitlab.com/gitlab-org/container-registry/blob/31e415038fed9a48435f0c041de173723820148e/registry/storage/driver/storagedriver.go#L198-198, but that doesn't support (un)wrapping, appending, etc. We can replace that with this in a followup.