cli: poor messaging on misconfigured duration flag

[nnchang]

If I have a cli with a duration flag that is not required and has no default, it will always panic with the message:

panic: time: invalid duration

this happens whether or not the user provides a value on the command line. Turns out this panic arises in the default-parsing code:

pkg/cli/flag/duration_flag.go

Lines 51 to 54 in b9b4547

	func (f DurationFlag) Default() interface{} {
	dur, err := f.Parse(f.DefaultStr())
	if err != nil {
	panic(err)

We should have better messaging to ensure developers aren't confused when the panic happens.