[Form] Improved exception message if the data class is not found

src/Symfony/Component/Form/FormConfigBuilder.php

@@ -193,7 +193,7 @@ class FormConfigBuilder implements FormConfigBuilderInterface
         self::validateName($name);
 
         if (null !== $dataClass && !class_exists($dataClass)) {
-            throw new InvalidArgumentException(sprintf('The data class "%s" is not a valid class.', $dataClass));
+            throw new InvalidArgumentException(sprintf('Class "%s" not found. Is the "data_class" form option set correctly?', $dataClass));
         }
 
         $this->name = (string) $name;