Modify the "unique" validator to accept additional property names to
narrow the space of rows searched for duplicates.
Example:
Consider `SiteUser` belongsTo `Site` via `siteId` foreign key.
Inside every site, the user email must be unique. It is allowed to
register the same email with multiple sites.
SiteUser.validateUniquenessOf('email', { scopedTo: ['siteId'] });
- change `statusCode` from 400 to 422
- nest `context` and `codes` inside `details`
- add `details.messages`
- reword the main error message
Remove the call to Error's constructor from ValidationError constructor,
because it's a no-op - Error's constructor creates a new
instance when called via `.call()`.