[core] return error message on invalid dob

[regisoc]

Brief summary of changes

When using /api/v0.0.3/candidates with an invalid date format, returns 400 Bad Requests/JSON errors.
This PR checks the date of birth and returns the correct format in the message ('yyyy-mm-dd').

Testing instructions (if applicable)

1. Log into to get a token:

curl -v -k -X 'POST' \
  'https://<hostname>/api/v0.0.3/login' \
  -H 'accept: application/json' \
  -H 'Content-Type: application/json' \
  -d '{
  "username": "<username>",
  "password": "<password>"
}'

2. Test for /api/v0.0.3/candidates endpoint:

curl -v -k -X POST 'https://<hostname>/api/v0.0.3/candidates' \
    -H "Authorization: Bearer <token>" \
    -d '{
        "Candidate": {
            "Project": "Rye",
            "Site": "Montreal",
            "DoB": "<a_date>",
            "Sex": "Female"
        }
    }'

Link(s) to related issue(s)

• Incorrect error message with invalid date format #8127

[regisoc]

@xlecours do I need a rebase to 24.1 for this PR? I am not entirely sure

[xlecours]

@xlecours do I need a rebase to 24.1 for this PR? I am not entirely sure

I am not sure actually. I would say, send a PR to 24.1 as well.
@driusan ?

[driusan]

Yes, 24.1 is the latest release, so it still gets bug fixes.

[regisoc]

Duplicated to #8279 for 24.1 fix

[driusan]

don't duplicate PRs, they'll get pulled forward eventually.. just rebase this one

[regisoc]

There are lots of changes for the rebase.
If that is ok, I will close this PR and keep the other one, the branch is already up-to-date with 24.1 on the other one.