Add field from server to set custom error messages for logistration fields

@AlasdairSwan The new output will look like this:
```
       {
            "method": "post",
            "submit_url": "/submit",
            "fields": [
                {
                    ...
                    "restrictions": {},
                    "errorMessages": {
                           "required": "This field is required!"
                    },
                },
            ]
        }
```