minor symfony#25492 improve FormType::getType exception message details (Nicals)

This PR was submitted for the master branch but it was squashed and merged into the 2.8 branch instead (closes symfony#25492).

Discussion
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improve FormType::getType exception message details

| Q             | A
| ------------- | ---
| Branch?       | master
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

This pull request improves the exception thrown when a Type is requested from Symfony\Component\Form\FormRegistry.

The same exception message was thrown if either the Type class wasn't found or the Type class did not implement the Symfony\Component\Form\FormTypeInterface.

This pull request adds some explaination of the reason why a type could not be found and helps application developpers to find the source of the Symfony\Component\Form\Exception\InvalidArgumentException thrown when getting a Type.

Commits
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945f236 improve FormType::getType exception message details

Author: nicolas-grekas

src/Symfony/Component/Form/FormRegistry.php

@@ -84,11 +84,14 @@ public function getType($name)
 
             if (!$type) {
                 // Support fully-qualified class names
-                if (class_exists($name) && in_array('Symfony\Component\Form\FormTypeInterface', class_implements($name))) {
-                    $type = new $name();
-                } else {
-                    throw new InvalidArgumentException(sprintf('Could not load type "%s"', $name));
+                if (!class_exists($name)) {
+                    throw new InvalidArgumentException(sprintf('Could not load type "%s": class does not exist.', $name));
                 }
+                if (!is_subclass_of($name, 'Symfony\Component\Form\FormTypeInterface')) {
+                    throw new InvalidArgumentException(sprintf('Could not load type "%s": class does not implement "Symfony\Component\Form\FormTypeInterface".', $name));
+                }
+
+                $type = new $name();
             }
 
             $this->resolveAndAddType($type);