Improve validation error messages

[bolshakov]

Imagin you have a fetcher with the following filter:

filter genres: :in, validates: { presence: true }

If you forget to pass this filter when fetching resource, the following error will be raised:

Genres should not be empty

To provide more user friendly feedback, error should be slightly more specific. Something like:

Filter by "genres" should not be empty. You can pass it using the following syntax: "?filter[genres_in]={genres}"