[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.
Would it make also sense to be smarter when no docroot is provided? By default, the value is `web/`, but we can probably do better.