minor #14231 [DependencyInjection] Show better error when the Yaml component is not installed (dosten)

This PR was squashed before being merged into the 2.3 branch (closes #14231).

Discussion
----------

[DependencyInjection] Show better error when the Yaml component is not installed

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

As the Yaml component is a suggested dependency, if someone try to use the `YamlFileLoader` or the `YamlDumper` without the component will get an ugly message:
```
PHP Fatal error:  Class 'Symfony\Component\Yaml\Dumper' not found in xxxx
```
With this PR the error will be an exception with the message: `Unable to dump the container as the Symfony Yaml Component is not installed.` for `YamlDumper` and `Unable to load YAML files service definitions as the Symfony Yaml Component is not installed.'` for `YamlFileLoader`

Commits
-------

870a299 [DependencyInjection] Show better error when the Yaml component is not installed

Author: fabpot

Dumper/YamlDumper.php

@@ -18,7 +18,6 @@
 use Symfony\Component\DependencyInjection\Parameter;
 use Symfony\Component\DependencyInjection\Reference;
 use Symfony\Component\DependencyInjection\Exception\RuntimeException;
-use Symfony\Component\DependencyInjection\ContainerBuilder;
 
 /**
  * YamlDumper dumps a service container as a YAML string.
@@ -31,20 +30,6 @@ class YamlDumper extends Dumper
 {
     private $dumper;
 
-    /**
-     * Constructor.
-     *
-     * @param ContainerBuilder $container The service container to dump
-     *
-     * @api
-     */
-    public function __construct(ContainerBuilder $container)
-    {
-        parent::__construct($container);
-
-        $this->dumper = new YmlDumper();
-    }
-
     /**
      * Dumps the service container as an YAML string.
      *
@@ -56,6 +41,14 @@ public function __construct(ContainerBuilder $container)
      */
     public function dump(array $options = array())
     {
+        if (!class_exists('Symfony\Component\Yaml\Dumper')) {
+            throw new RuntimeException('Unable to dump the container as the Symfony Yaml Component is not installed.');
+        }
+
+        if (null === $this->dumper) {
+            $this->dumper = new YmlDumper();
+        }
+
         return $this->addParameters()."\n".$this->addServices();
     }
 

Loader/YamlFileLoader.php

@@ -17,6 +17,7 @@
 use Symfony\Component\DependencyInjection\Definition;
 use Symfony\Component\DependencyInjection\Reference;
 use Symfony\Component\DependencyInjection\Exception\InvalidArgumentException;
+use Symfony\Component\DependencyInjection\Exception\RuntimeException;
 use Symfony\Component\Config\Resource\FileResource;
 use Symfony\Component\Yaml\Parser as YamlParser;
 
@@ -269,6 +270,10 @@ private function parseDefinition($id, $service, $file)
      */
     protected function loadFile($file)
     {
+        if (!class_exists('Symfony\Component\Yaml\Parser')) {
+            throw new RuntimeException('Unable to load YAML config files as the Symfony Yaml Component is not installed.');
+        }
+
         if (!stream_is_local($file)) {
             throw new InvalidArgumentException(sprintf('This is not a local file "%s".', $file));
         }