Validation messages translations

[s12v]

Attempt to translate validation messages using Play i18n

At the moment, validation messages are not very helpful:

[
  {
    "messages": [
      "error.maxLength"
    ],
    "args": [
      "2"
    ]
  }
]

This PR changes it to:

{
  "errors": [
    {
      "reference": "⌿definitions⌿VendorQuery⌿language",
      "message": "Maximum length is 2"
    }
  ]
}

TODO:

• [x] Include parameter name, so one would not have to figure out which length :)
• [x] Fix tests
• [x] Update documentation

@slavaschmidt, @gipeshka, what do you think?

[slavaschmidt]

Looks good 👍

[s12v]

@slavaschmidt, docs updated, ready for review

[s12v]

@slavaschmidt, maybe we can have 0.3.0 for this? It's not a big change, but the validation response format has been changed.

[slavaschmidt]

Let's make it 0.2.3

The change from 0.2 to 0.3 would be something big, like breaking change with play 2.5 support for 0.2