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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.01%. Comparing base (16abe6a) to head (6586970).
Report is 31 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #360      +/-   ##
==========================================
+ Coverage   95.68%   97.01%   +1.33%     
==========================================
  Files          14       15       +1     
  Lines         950     1173     +223     
==========================================
+ Hits          909     1138     +229     
+ Misses         41       35       -6     

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