[symfony/mailjet-mailer] Fix invalid mailjet error managment

[alamirault]

Q	A
Branch?	5.4
Bug fix?	yes
New feature?	no
Deprecations?	no
Tickets	Fix #45420
License	MIT
Doc PR

When mailjet API status code is not 200, response is structured (see-below) and is not currently well managed.

Example of bad request response:

{
  "Messages": [
    {
      "Status": "error",
      "Errors": [
        {
          "ErrorIdentifier": "8e28ac9c-1fd7-41ad-825f-1d60bc459189",
          "ErrorCode": "mj-0005",
          "StatusCode": 400,
          "ErrorMessage": "The To is mandatory but missing from the input",
          "ErrorRelatedTo": ["To"]
        }
      ]
    }
  ]
}

This PR fix Undefined index: Message and add unit tests.

[carsonbot]

Hey!

I think @tcheymol has recently worked with this code. Maybe they can help review this?

Cheers!

Carsonbot

[fabpot]

Thank you @alamirault.