Error are jsonify and should have the same schema

Error will be a dict:

{
  'error': 'This is a message explaing the error',
  'field': 'field_name'
}

if the error is on a 'filter' or 'sort' parameters (query string) the key 'field' will be set with the name of the faulty parameters

Fix #15

Coverage decreased (-2.08%) to 59.569% when pulling e9725500616471f8ec476347d8a084e08ed92498 on jysonify-error into 731eaab7f3bc9ef913cf5641ae4e3c8abdd2f29f on master.