[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` 
An error message is already printed in `createPhpProcessBuilder()` when it returns `null`.