linter: enable nilerr

A number of cases looked suspicious, so I marked them with `FIXME`s to
leave some breadcrumbs.

Signed-off-by: Valentin Rothberg <vrothberg@redhat.com>

pkg/errorhandling/errorhandling.go

@@ -28,7 +28,7 @@ func JoinErrors(errs []error) error {
 
 	finalErr := multiE.ErrorOrNil()
 	if finalErr == nil {
-		return finalErr
+		return nil
 	}
 	return errors.New(strings.TrimSpace(finalErr.Error()))
 }