Improve model validation error for GraphQL

[vincent-pochet]

For GraphQL mutation, we should improve the way validation errors are sent to the front.

It will allow the front end to properly handle translations and field errors

In order to do to so, in case of unprocessable_entity error, an extension property of the GraphQL error will give the full detail of what went wrong when processing the mutation on the model.

Example of payload:

{
  "customer_id": ["already_taken"],
  "country": ["invalid"]
}