Rest – validation error responses in REST API

I'm designing a RESTful API and wondering what's the best format for validation error messages.

For example, my account creation endpoint accepts a JSON object:

user: {
  first_name: string,
  last_name: string,
  address: {
    street: string,
    city: string,
    zip_code: string
  }
}

My responses will be in the following format:

{
    code: 400,  // HTTP code
    message: "Validation failed",  // general message
    type: "validation_failed",  // there are other types of errors as well
    errors: WHAT_DO_I_SHOW_HERE
}

I have several choices for validation error messages:

Format 1

errors: {
  last_name: "First name is required",
  address: {
    zip_code: "ZIP code is invalid"
  }
}

or flatten the errors as in Format 2

errors: {
  last_name: "First name is required",
  "address.city": "City is required",
  "address.zip_code": "ZIP code is invalid"
}

or use an array, where each element can have field name, error code, error message, nested errors, etc.

errors: [
  {
    field: "first_name",
    message: "First name is required",
  },
  {
    field: "address",
    errors: [
      {
        field: "zip_code",
        message: "ZIP code is invalid"
      }
    ]
  }
]

or

errors: [
  {
    field: "first_name",
    message: "First name is required",
  },
  {
    field: "address.zip_code",
    message: "ZIP code is invalid"
  }
]

Apparently the array format is more flexible since field name is optional, so it can accommodate errors related to a combination of multiple fields (e.g., end time of a time interval must be after the begin time). But my question is, which one would be easier for API users to consume?