Error are jsonify and should have the same schema

[MoiTux]

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

[coveralls]

Coverage decreased (-2.08%) to 59.569% when pulling e972550 on jysonify-error into 731eaab on master.