[Good case] Handle errors in defer

I hate named returns too :)

Well, one case are good and should be allowed by linter I suppose.

``` go
func f() (retErr error) {
	err := errors.New("foo")
	defer func() {
		if retErr == nil { // <-- analyse and replace returning error
			retErr = errors.New("bar")
		}
	}()
	return err
}
```