Update kubectl.GetPrinter to not return both nil for the printer and nil for the error on wide format

[juanvallejo]

Problem

Right now, printer helpers such as cmdutil.PrinterForCommand call kubectl.GetPrinter with the output format specified by a --output flag, in order to retrieve the right printer for that format.

When kubect.GetPrinter is called with a wide format, nil is returned for both the printer and the error. This has inevitably led to panics throughout kubectl commands whenever code that calls kubectl.GetPrinter (either directly or indirectly through cmdutil.PrinterForCommand`) attempts to use the retrieved printer if no errors are returned. For example:

// $ kubectl ... -o wide

outputFormat := cmdutil.GetFlagString(cmd, "output")
printer, generic, err := kubectl.GetPrinter(outputFormat, ...)
if err != nil {
    // handle error
    return
}

// any code here assumes no error AND that the printer exists, however since both the printer and the 
// error were nil for the "wide" outputFormat, the line below will cause a panic
printer.PrintObj(info.Object, out)

Commands like kubectl create and kubectl apply get around this by only supporting the name output format, however when expanding these commands to eventually use all supported output formats by resource printers (json, yaml, custom-columns, etc...), this becomes an issue.

Proposal

1. A solution to this could either be to actually return an error in kubect.GetPrinter if the returned printer will be nil

• which would then allow existing helpers such as cmdutil.PrinterForCommand to handle this and return a NewHumanReadablePrinter with the right PrintOptions for example,

2. or to simply return a NewHumanReadablePrinter in kubectl.GetPrinter any time the output format specified == wide.

cc @fabianofranz @pwittrock @adohe @deads2k

[fabianofranz]

@kubernetes/sig-cli @kubernetes/kubectl