Added property name in draft-3 required error (#432)

Backport modifications:
 * Change error syntax to use the old pre-6.0.0 API

src/JsonSchema/Constraints/UndefinedConstraint.php

@@ -142,7 +142,13 @@ protected function validateCommonProperties(&$value, $schema = null, JsonPointer
  } elseif (isset($schema->required) && !is_array($schema->required)) {
  // Draft 3 - Required attribute - e.g. "foo": {"type": "string", "required": true}
  if ($schema->required && $value instanceof self) {
- $this->addError($path, 'Is missing and it is required', 'required');
+ $propertyPaths = $path->getPropertyPaths();
+ $propertyName = end($propertyPaths);
+ $this->addError(
+ $path,
+ 'The property ' . $propertyName . ' is required',
+ 'required'
+ );
  }
  }
  }