Improve collection of validation errors

As mentioned in PR #433 I had a more polished version of the PR but was not yet able to push it. I was a little bit surprised to see the merge that quickly. So I hope there is still some time to ameliorate the merge with this PR.

Notable changes:
* rename ``Grape::Exceptions::Validations`` to ``Grape::Exceptions::ValidationErrors`` for clarity 
* make it necessary to supply :param when raising Grape::Exception::Validation
* group validation errors by parameter