improve collection of validation errors

rename Grape::Exceptions::Validations to Grape::Exceptions::ValidationError for clarity
make it necessary to supply :param when raising Grape::Exception::Validation
use rails as inspiration for the structure of the validation errors
Merge remote-tracking branch 'upstream/master'

Conflicts:
	CHANGELOG.md
	spec/grape/validations/coerce_spec.rb
collect the validation errors objects instead of their message when calling #errors
update changelog and readme
Merge pull request #462 from ninech/master

Improve collection of validation errors