fs: make fs.watch error message more useful

PR-URL: #5616
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Trevor Norris <trev.norris@gmail.com>

Author: jasnell

lib/fs.js

@@ -1382,7 +1382,10 @@ function FSWatcher() {
   this._handle.onchange = function(status, event, filename) {
     if (status < 0) {
       self._handle.close();
-      const error = errnoException(status, `watch ${filename}`);
+      const error = !filename ?
+          errnoException(status, 'Error watching file for changes:') :
+          errnoException(status,
+                         `Error watching file ${filename} for changes:`);
       error.filename = filename;
       self.emit('error', error);
     } else {