errors.Errorf doesn't support %w format

[bivas]

Issue I noticed is when trying errors.Errorf("reached 0 %w", Fail): This is actually doesn't support Go 1.13 error wrapping format (output was reached 0 %!w(*failure.FailError=&{}))

Created the following snippet to test it:

var (
	Fail = &FailError{}
)

type FailError struct{}

func (f FailError) Error() string {
	return "This is a fail error"
}

func FailR(depth int) error {
	if depth == 0 {
		return errors.Errorf("reached 0 %w", Fail) // This works fine: fmt.Errorf("reached 0 %w", Fail)
	}
	return errors.Wrap(FailR(depth-1), fmt.Sprintf("Failed stack (%d)", depth))
}