Show invalid message for select fields

[elio2t]

Fixes #270

It can be useful to display invalid messages for select fields when their validation depends on another field.

Before:

After:

For this error 422:

{
    "detail": {
        "form": [
            {
                "type": "text",
                "loc": [
                    "text_field"
                ],
                "msg": "Text field error"
            },
            {
                "type": "select",
                "loc": [
                    "select_single"
                ],
                "msg": "Select field error"
            }
        ]
    }
}

We could go further and display the border in red like the text field, but that doesn't seem trivial.

[codecov]

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