strconv.Unquote could wrongly return a nil error on error.

R=rsc, gri
CC=golang-dev
https://golang.org/cl/773041

src/pkg/strconv/quote.go

@@ -239,7 +239,7 @@ func Unquote(s string) (t string, err os.Error) {
 		return s, nil
 	}
 	if quote != '"' && quote != '\'' {
-		return "", err
+		return "", os.EINVAL
 	}
 
 	var buf bytes.Buffer