[FrameworkBundle] improve server commands feedback

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | kind of
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #12153
| License       | MIT
| Doc PR        |

* display a message when `server:start` is executed and the PCNTL
  extension is not loaded
* print instructions about how to terminate the `server:run` command