pkg: minor cleanup

remove some redundant .String()
change variable name to make it os.Error-proof

R=golang-dev, iant
CC=golang-dev
https://golang.org/cl/5302075

src/pkg/archive/zip/struct.go

@@ -60,10 +60,10 @@ type directoryEnd struct {
 	comment            string
 }
 
-func recoverError(err *os.Error) {
+func recoverError(errp *os.Error) {
 	if e := recover(); e != nil {
-		if osErr, ok := e.(os.Error); ok {
-			*err = osErr
+		if err, ok := e.(os.Error); ok {
+			*errp = err
 			return
 		}
 		panic(e)

src/pkg/big/int_test.go

@@ -536,7 +536,7 @@ func TestScan(t *testing.T) {
 		buf.Reset()
 		buf.WriteString(test.input)
 		if _, err := fmt.Fscanf(&buf, test.format, x); err != nil {
-			t.Errorf("#%d error: %s", i, err.String())
+			t.Errorf("#%d error: %s", i, err)
 		}
 		if x.String() != test.output {
 			t.Errorf("#%d got %s; want %s", i, x.String(), test.output)

src/pkg/big/rat_test.go

@@ -112,7 +112,7 @@ func TestRatScan(t *testing.T) {
 		_, err := fmt.Fscanf(&buf, "%v", x)
 		if err == nil != test.ok {
 			if test.ok {
-				t.Errorf("#%d error: %s", i, err.String())
+				t.Errorf("#%d error: %s", i, err)
 			} else {
 				t.Errorf("#%d expected error", i)
 			}