chore: Enable nilerr, nilnesserr

[alexandear]

What type of PR is this?

/kind cleanup

What this PR does / why we need it:

Enable nilerr and nilnesserr linters to find places where returning nil may be wrong.
nilerr finds places that return nil even if it checks that the error is not nil.
nilnesserr reports constructs that checks for err != nil, but returns a different nil value error.

Special notes for your reviewer:

The nilerr linter founds two places where err is not nil, but returned value is nil:

1. In the clustersReconciler.Reconcile:

	err := c.localClient.Get(ctx, req.NamespacedName, cluster)
	if client.IgnoreNotFound(err) != nil {
		return reconcile.Result{}, err
	}
	log.V(2).Info("Reconcile MultiKueueCluster")

	if err != nil || !cluster.DeletionTimestamp.IsZero() {
		c.stopAndRemoveCluster(req.Name)
		return reconcile.Result{}, nil
	}

Returning nil is expected because err is NotFound and we ignore it. To silence the linter, I changed nil to client.IgnoreNotFound(err).

2. In the JobSet.RunWithPodSetsInfo:

	if err := podset.Merge(&template.ObjectMeta, &template.Spec, info); err != nil {
		return nil
	}

I suspect there is a bug here, and I changed to return err.

Does this PR introduce a user-facing change?

NONE

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[k8s-ci-robot]

Hi @alexandear. Thanks for your PR.

I'm waiting for a kubernetes-sigs member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[mbobrovskyi]

/ok-to-test

[gabesaba]

/assign

[gabesaba]

I think that returning nil here is intentional, as either the cluster is deleted, or not found (since the only err that will cause us to hit this branch is NotFound). If we return an error, I think we will repeatedly keep trying to process this deleted or not found cluster.

Is there a possible refactor that will make this more readable, and make the linter happy?