Fix kubectl -o error message:
Before this change:
```shell
kubectl get pods -o foo
error: unable to match a printer suitable for the output format "" and the options specified: &get.PrintFlags{JSONYamlPrintFlags:(*genericclioptions.JSONYamlPrintFlags)(0x23aa610), NamePrintFlags:(*genericclioptions.NamePrintFlags)(0xc42058b4e0), TemplateFlags:(*printers.KubeTemplatePrintFlags)(0xc4206765e0), CustomColumnsFlags:(*printers.CustomColumnsPrintFlags)(0xc420676620), HumanReadableFlags:(*get.HumanPrintFlags)(0xc4204eb180), NoHeaders:(*bool)(0xc4206fefbc), OutputFormat:(*string)(0xc42058b4d0)}
```
After this change:
```shell
Kubectl get pods -o foo
error: unable to match a printer suitable for the output format "aaa", allowed formats are: custom-columns,custom-columns-file,go-template,go-template-file,json,jsonpath,jsonpath-file,name,template,templatefile,wide,yaml
```
