[2.5][FrameworkBundle] Fix server run in case the router script does not exist

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | 
| License       | MIT

I've also added the use of `Process\PhpExecutableFinder` 
I current implementation, if `createPhpProcessBuilder` fails, it returns `1`. The line 102 expects a `ProcessBuilder` : 

```
$builder = $this->createPhpProcessBuilder($input, $output, $env);
```

I prefer to return `null` in case no builder is created. I suppose the `return 1` is a wrong copy/paste when this code was inside the `Command::execute` method and where it was valid