os: make write_file() return an optional

vlib/os/os.v

@@ -863,9 +863,9 @@ pub fn home_dir() string {
 }
 
 // write_file writes `text` data to a file in `path`.
-pub fn write_file(path, text string) {
+pub fn write_file(path, text string) ?{
 	mut f := os.create(path) or {
-		return
+		return error(err)
 	}
 	f.write(text)
 	f.close()