error messages can now be presented

This PR is a proposal (and misses specs & co) to present error messages like started to discuss [here](https://github.com/tim-vandecasteele/grape-swagger/pull/110).
It will be possible to do some thing like this:


```ruby

module  API
  module Entities
    class Error < Grape::Entity
      expose :error, documentation: { type: 'string', required: true }
      expose :code, documentation: { type: 'integer', required: true }
      expose :detail, documentation: { type: 'string', required: true }
      expose :href, documentation: { type: 'url', required: true }
      expose :rel, documentation: { type: 'string', required: true }
      expose :time, documentation: { type: 'datetime', required: true }

      def rel
        'self'
      end

      def href
        options[:env]['api.endpoint'].request.url
      end

      def time
        Time.now.utc
      end
    end
  end
end

error!({ error: 'Not Found',
            detail: "Some more information",
            with: API::Entities::Error
           }, 404)

```

This PR requires the upcomming version of  `grape-entity` .

This is swallowing a useful exception, and seems to only serve specs. In some conditions you're going to get unexpected results and will have no way of knowing why. Specs should be fixed instead of this.