title: Mongoose - Return error in 'pre' middleware
body: <p>How can I send a custom error message if my validation fails in <code>schema.pre('save')</code>? For example, if I have a chat feature, where you create a new conversation, I want to check if a conversation with the given participants already exists, so I can do:</p>
  
  <pre><code>ConversationSchema.pre('save', function(next, done) {
      var that = this;
      this.constructor.findOne({participants: this.participants}).then(function(conversation) {
          if (conversation) {
              // Send error back with the conversation object
          } else {
              next();
          }
      });
  });
  </code></pre>