replace juju/errors because of CNCF license scan report

[mars1024]

Just as the newest license scan report from CNCF, juju/errors will not be suitable to be used in our repos.
We should find another way to replace it.

[mars1024]

Err in juju/errors is just like below

func (e *Err) Error() string {
	// We want to walk up the stack of errors showing the annotations
	// as long as the cause is the same.
	err := e.previous
	if !sameError(Cause(err), e.cause) && e.cause != nil {
		err = e.cause
	}
	switch {
	case err == nil:
		return e.message
	case e.message == "":
		return err.Error()
	}
	return fmt.Sprintf("%s: %v", e.message, err)
}

So I will replace it with fmt.Errorf ..