Ensure wrapping nil is still nil

[gabrielf]

This PR makes it possible to write code like:

func myCode() error {
    // …

    // Next line would previously always return a non nil error
    return errors.Wrap(libraryCode(), 0)
}

Accessing the stack trace becomes harder. Previously this could be done with errors.Wrap(err, 0).ErrorStack() but now that Wrap returns error you would have to write errors.Wrap(err, 0).(*errors.Error).ErrorStack(). To simplify access of the stack trace a new top level function has been added so that you can do: errors.ErrorStack(err) to access the stack trace.

If there is no stack trace