Improve `Schema.validate()` exception message
Improve `Schema.validate()` exception message

sirex/datapackage-py,okfn/datapackage-model-py,datapackages/datapackage-py,okfn/datapackage-py,okfn/datapackage-model-py,datapackages/datapackage-py,sirex/datapackage-py,okfn/datapackage-py