…xist (neeckeloo)

This PR was merged into the 4.2-dev branch.

Discussion
----------

[Messenger] Improved message when handler class does not exist

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

**Problem:**

When defining a non existing messenger handler class in the `services.yml` config file, we encounter this confusing error message:

```
services:
    App\Handler\NonExistentHandler:
        tags: [messenger.message_handler]
```

```
PHP Fatal error:  Uncaught Symfony\Component\Debug\Exception\FatalThrowableError: Argument 1 passed to Symfony\Component\Messenger\DependencyInjection\MessengerPass::guessHandledClasses() must be an instance of ReflectionClass, null given, called in /app/vendor/symfony/messenger/DependencyInjection/MessengerPass.php on line 93 in /app/vendor/symfony/messenger/DependencyInjection/MessengerPass.php:189
Stack trace:
    #0 /app/vendor/symfony/messenger/DependencyInjection/MessengerPass.php(93): Symfony\Component\Messenger\DependencyInjection\MessengerPass->guessHandledClasses(NULL, 'App\\Application...')
    #1 /app/vendor/symfony/messenger/DependencyInjection/MessengerPass.php(74): Symfony\Component\Messenger\DependencyInjection\MessengerPass->registerHandlers(Object(Symfony\Component\DependencyInjection\ContainerBuilder), Array)
    #2 /app/vendor/symfony/dependency-injection/Compiler/Compiler.php(95): Symfony\Component\Messenger\DependencyInjection\MessengerPass->process(Object(Symfony\Component\DependencyInjection\ContainerBuilder))
    #3 / in /app/vendor/symfony/messenger/DependencyInjection/MessengerPass.php on line 189
```

**Proposal:**

We can throw a more relevant exception (RuntimeException) in this case to help the developer to have a better understanding of the issue.

```Invalid service "App\Handler\NonExistentHandler": class "App\Handler\NonExistentHandler" does not exist.```

Commits
-------

6ab9274 Improve error message when defining messenger handler class that does not exists