kubectl exec: return descriptive error message when multi resource passed

`kubectl exec` command supports getting files as inputs. However,
if the file contains multiple resources, it returns unclear error message;
`cannot attach to *v1.List: selector for *v1.List not implemented`.

Since `exec` command does not support multi resources, this PR
handles that and returns descriptive error message earlier.

staging/src/k8s.io/kubectl/pkg/cmd/exec/exec.go

@@ -33,6 +33,7 @@ import (
 	restclient "k8s.io/client-go/rest"
 	"k8s.io/client-go/tools/remotecommand"
 
+	"k8s.io/apimachinery/pkg/api/meta"
 	cmdutil "k8s.io/kubectl/pkg/cmd/util"
 	"k8s.io/kubectl/pkg/cmd/util/podcmd"
 	"k8s.io/kubectl/pkg/polymorphichelpers"
@@ -310,6 +311,10 @@ func (p *ExecOptions) Run() error {
 			return err
 		}
 
+		if meta.IsListType(obj) {
+			return fmt.Errorf("cannot exec into multiple objects at a time")
+		}
+
 		p.Pod, err = p.ExecutablePodFn(p.restClientGetter, obj, p.GetPodTimeout)
 		if err != nil {
 			return err