Allows custom message for each path 

Allows custom message using the unique path, for example:

```javascript
mongoose.Schema({
        username: {
            type: String,
            unique: 'Username is already used.'
        },
        email: {
            type: String,
            index: true,
            unique: 'It already exists.'
        },
        password: {
            type: String
        }
    });
```