Reformat /validate endpoint response

[csc-jm]

Description

Currently validating an xml file via /validate returns a 200 response whether the file is valid or not. However, upon changes done in #630, it was determined that reformatting the response according to JSON Problem specification (https://www.rfc-editor.org/rfc/rfc7807) when the file is in fact invalid. This will require changes in the front-end as well.

DoD (Definition of Done)

The response after an invalid file gets POSTed to the /validate endpoint should instead look e.g. something like this:

{
   "type": "",
   "title": "Bad Request",
   "detail": "Faulty XML file provided."
   "validation-failures": [ {
                         "reason": "Mismatched tag on line 1.",
                         "position": "/ELEMENT_SET"
                       },
                       {
                         "reason": "something else",
                         "position": "/"}
                     ]
   "status": 400,
   "instance": "/validate"
}

Testing

Unit and integration