io/ioutil: make WriteFile notice errors from Close()

WriteFile should not successfully return if the file's
Close call failed.

Fixes #5644.

R=golang-dev, kr, r
CC=golang-dev
https://golang.org/cl/10067043

src/pkg/io/ioutil/ioutil.go

@@ -78,10 +78,12 @@ func WriteFile(filename string, data []byte, perm os.FileMode) error {
 		return err
 	}
 	n, err := f.Write(data)
-	f.Close()
 	if err == nil && n < len(data) {
 		err = io.ErrShortWrite
 	}
+	if err1 := f.Close(); err == nil {
+		err = err1
+	}
 	return err
 }