Fail when models are partially initialised and add a new more informative health api entry

[lfoppiano]

This PR addresses the issue in #1372.

1. We improved the /api/isalive to return false when a mode is not working (e.g. jep initialisation error)
2. We added a new /api/health that returns more detailed information of the status of grobid

{
  "initialized": false,
  "ready": false,
  "pool": {
    "initialized": true,
    "active": 0,
    "idle": 0,
    "maxActive": 10
  },
  "models": {
    "loaded": {
      "table": "wapiti",
      "name-citation": "wapiti",
      "date": "wapiti",
      "figure": "wapiti",
      "affiliation-address": "wapiti",
      "segmentation": "wapiti",
      "reference-segmenter": "wapiti",
      "funding-acknowledgement": "wapiti",
      "fulltext": "wapiti",
      "name-header": "wapiti",
      "citation": "wapiti"
    },
    "failed": {
      "header": "JEP initialization failed"
    },
    "totalLoaded": 11,
    "totalFailed": 1
  },
  "grobidHomeConfigured": true,
  "initializationError": "Models failed to load: header"
}