`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.
Kubernetes-commit: 832644f0b38d536be7a5adce9bc62b0902710091