Improve document validation formatting

[olzraiti]

• [ ] Drop support for document endpoint validationErrorFormat.jsonPath, because:
  • It's not really JSON path, because it doesn't start with "$".
  • JSON path is misleading to use, because it can target multiple items.
• [ ] Remove "errors" wrapper in document validation responses. Should use an array instead. Reasons:
  • Using the object notation, it would be impossible to target a property that has the name "errors"
  • Unnecessary; adds to complexity.