doc: add error check to fs example

Previously, the err passed to the callback of fs.open() was not checked.

PR-URL: https://github.com/nodejs/node/pull/18681
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>

doc/api/fs.md

@@ -1397,6 +1397,7 @@ fs.open('myfile', 'wx', (err, fd) => {
 fs.exists('myfile', (exists) => {
   if (exists) {
     fs.open('myfile', 'r', (err, fd) => {
+      if (err) throw err;
       readMyData(fd);
     });
   } else {