This is a general error that came out of the discussions surrounding reformatting/rewriting all the collision/conflict errors
**Acceptance criteria:**
- **Current messages it will apply to**: patch-result-label-description-duplicate, duplicate-label, label-description-duplicate, label-description-same-value, sitelink-conflict
- **HTTP status code**: 422 (might be 409 for all or some of the current errors)
- **Error code**: data-policy-violation
- **Error message**: Edit violates data policy
- **Context**: { Collision,'violated-policy': 'unique-property-label|unique-label-description|TODO-something-smart-for-label-desc-same|unique-sitelink-target', and more, see below }
Current fav: option 1 of the ones below
1. nested policy context
{ 'violated-policy': 'unique-label-description',
'violation-context': { 'language-code': 'foo', 'conflicting-item': ...
2. policy dependent context fields
{ 'violated-policy': 'unique-sitelink-target',
'site-id': 'foo'
3. generic field
{ 'violated-policy': 'unique-label-description',
value already exists: 'label|'target-of-violating-data': '/label-description|sitelink' }s/en',
'conflicting-resource': 'Q123' (special case, for label-description-same the ID of self)