minor symfony#13431 [Form] Improved exception message if the data class is not found (fabpot)

This PR was merged into the 2.3 branch.

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

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | symfony#12359, symfony#12367
| License       | MIT
| Doc PR        | -

Commits
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4145836 [Form] Improved exception message if the data class is not found

Author: fabpot

src/Symfony/Component/Form/FormConfigBuilder.php

@@ -193,7 +193,7 @@ public function __construct($name, $dataClass, EventDispatcherInterface $dispatc
         self::validateName($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;