throw error if notes dir has not been created yet

lib/node.js

@@ -36,6 +36,8 @@ export const Node = {
   },
 
   getMarkdownFolder: function() {
-    return path.join(process.cwd(), 'posts')
+    const notesDir = path.join(process.cwd(), 'notes')
+    if (!Node.isFile(notesDir)) throw new Error("The notes directory has not been created yet")
+    return notesDir
   }
 }