Fields: Add a clear error message when length is missing

[wellingguzman]

Describe the bug
Creating a field that requires a valid length it throws an mysql error.

To Reproduce
Steps to reproduce the behavior:

1. Make the following request

POST https://example.com/_/fields/test_collection

{
    "sort": null,
    "field": "rijk",
    "type": "CSV",
    "interface": "checkboxes",
    "default_value": null,
    "options": {},
    "readonly": false,
    "required": false,
    "unique": false,
    "note": "leuk",
    "hidden_input": false,
    "hidden_list": false,
    "length": null,
    "validation": null,
    "collection": "test_collection"
}

2. Get the mysql error

[wellingguzman]

Closed by 9268217 and dc87574