Enhanced errors schema for rich error display in deposit form

[slint]

To display rich errors resulting from the configured checks, we need to extend the current schema of the errors key returned in draft REST API responses, so that it looks something like:

{
   ...
   "errors": [
       {
           "field": "metadata.funding",
           "messages": ["At least one EC-funded award should be present."],
           "severity": "info",  // failure, warning, info
           "context": {
               "community": "<community_id>",
               "check": "<check_id>",
           }
       },
       {
           "field": "metadata.creators",
           "messages": ["Authors must use persistent identifiers (ORCiD, GND, etc.). 'Nielse, Lars Holm' and 'Ioannidis, Alex' are missing ORCiDs."],
           "severity": "warning",  // failure, warning, info
           "context": {
               "community": "<community_id>",
               "check": "<check_id>",
           }
       }
   ],

   ...
   "ui": {
      "communities": {
          "<community_id>": {...slug, title, etc...},
      },
      "checks": {
          "<check_id>": {...title, links, description, etc...},
      } 
   }
}