built-in server: exit when docroot does not exist

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.
Merge branch '2.3' into 2.4

* 2.3:
  Fixed relative redirects for ambiguous paths
  [BrowserKit] Fix browser kit redirect with ports
  [TwigBridge] [Form] Fixed some extra empty spaces
  Plural fix
  removed some .gitattributes that should have been removed a lot time ago
  [DependencyInjection] fixed missing 'factory-class' attribute in XmlDumper output
  fixed whitespace in Twig form template
  built-in server: exit when docroot does not exist

Conflicts:
	src/Symfony/Bridge/Twig/Resources/views/Form/form_div_layout.html.twig