[Bundle][FrameworkBundle] built-in server: exit when docroot does not exist

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #6496
| License       | MIT
| Doc PR        | 

When the server:run command is run with an invalid document root
directory (for example, when being in the app directory and not
changing the document root to ../web/), the command crashes on Windows
with a 267 exit code. On Linux, the server starts but just publishes
internal server errors.
I'm not sure. We can possibly detect the document root based on the kernel's root directory or something like that. But then, the default value somehow depends on the context in which the command is execute (i. e. the current working directory). I'm not sure if that is more confusing at all since the behaviour wouldn't be the same when you explicitly pass a directory.