When using daphne, if some process is already listening on our preferred port, `runserver` does not exit with a failure code.

[offby1]

if some server process is already listening on our preferred port, runserver does not exit with a failure code, but instead keeps running (even though the underlying daphne process has logged an error complaining about EADDRINUSE).

To reproduce the problem:

• clone the "repro-daphne-trouble" repository, and cd to its top-level directory (i.e., the folder that holds README.md)
• install uv
• run uv run python manage.py runserver; note that it's working fine
• in another window, again run uv run python manage.py runserver. This time, you'll see Listen failure: Couldn't listen on 127.0.0.1:8000: [Errno 48] Address already in use., but the process doesn't exit. The "Listen failure" message is expected, but the bug is that the process should exit with a nonzero status.

Notes:

• If, instead of uv run python manage.py runserver, you run uv run daphne repro.asgi:application (which is roughly equivalent), it works fine: the first process starts, and the second one both reports 2026-02-08 21:32:24,523 CRITICAL Listen failure: Couldn't listen on 127.0.0.1:8000: [Errno 48] Address already in use. and exits with status 1.
• If you check out the latest commit before adding daphne and try running two copies of "runserver", they work fine (i.e., the second process exits with a status of 1). So the problem is in some interaction with "runserver" and "daphne".
• There was a similar-seeming bug in daphne (Exits with 0, instead of nonzero, when it cannot bind to the specified port #552) that might have caused a similar problem, but it's irrelevant since it's been fixed for at least a year.