To upgrade list of each database given information about one particular error. Each dictionary can contain various keys depending on the type of error but you will always get the reason and the message keys as below:
reason: the error type
message: a human friendly message
For example you can see below code.

{
  "failures": [
    {
      "expected": [
        "6.0",
        "6.1",
        "7.0",
        "8.0",
      ],
      "message": "Invalid value \"5.0\"",
      "reason": "TARGET:INVALID",
      "value": "5.0"
    },
    {
      "code": "M123456-abcxyz",
      "message": "Can not find contract M123456-abcxyz",
      "reason": "CONTRACT:NOT_FOUND"
    }
  ]
}

Note- It allows a database to be upgraded without resorting the html form , for more you can visit below link-
https://upgrade.odoo.com
Although the database will follow the same process described on form link given above.