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.
bug #11545 [Bundle][FrameworkBundle] built-in server: exit when docroot does not exist (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
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[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.

Commits
-------

f143254 built-in server: exit when docroot does not exist