Add conformance tests for errors format

[Artifizer]

Need to develop error detection conformance tests:

1. Errors must be structured and return line number, json document string, error message
2. GTS errors must have the same message across various tools - gts-python, gts-rust, gts-go, gts-ts, gts-csharp

Example of bad message:

% ./target/release/gts validate-json --path .gts-spec-local/examples 
{
  "ok": false,
  "files": 64,
  "documents": 80,
  "gts_entities": 80,
  "schemas": 31,
  "instances": 49,
  "issues": [
    {
      "file": "/Users/aandreev/git/github/gts-spec/examples/events/types/gts.x.core.events.type.v1~x.core.audit.type.v1~.schema.json",
      "stage": "derived-type",
      "message": "Schema 'gts.x.core.events.type.v1~x.core.audit.type.v1~' is not compatible with base 'gts.x.core.events.type.v1~': derived schema 'gts.x.core.events.type.v1~x.core.audit.type.v1~' is not included in base 'gts.x.core.events.type.v1~': Schema at '$.type' changes type incompatibly from any to \"string\""
    },
    {
      "file": "/Users/aandreev/git/github/gts-spec/examples/events/types/gts.x.core.events.type.v1~x.core.audit.type.v1~x.core.iam.settings_changed.v1~.schema.json",
      "stage": "derived-type",
      "message": "Schema 'gts.x.core.events.type.v1~x.core.audit.type.v1~' is not compatible with base 'gts.x.core.events.type.v1~': derived schema 'gts.x.core.events.type.v1~x.core.audit.type.v1~' is not included in base 'gts.x.core.events.type.v1~': Schema at '$.type' changes type incompatibly from any to \"string\""
    }
  ]
}