[Embeddables] Improved exception message when embeddables is missing 'class' attribute.

lib/Doctrine/ORM/Mapping/ClassMetadataFactory.php

@@ -170,6 +170,10 @@ class ClassMetadataFactory extends AbstractClassMetadataFactory
                     continue;
                 }
 
+                if (!isset($embeddableClass['class'])) {
+                    throw MappingException::missingEmbeddedClass($property);
+                }
+
                 if (isset($this->embeddablesActiveNesting[$embeddableClass['class']])) {
                     throw MappingException::infiniteEmbeddableNesting($class->name, $property);
                 }

lib/Doctrine/ORM/Mapping/MappingException.php

@@ -105,6 +105,16 @@ class MappingException extends \Doctrine\ORM\ORMException
         return new self("The association mapping '$fieldName' misses the 'sourceEntity' attribute.");
     }
 
+    /**
+     * @param string $fieldName
+     *
+     * @return MappingException
+     */
+    public static function missingEmbeddedClass($fieldName)
+    {
+        return new self("The embed mapping '$fieldName' misses the 'class' attribute.");
+    }
+
     /**
      * @param string $entityName
      * @param string $fileName