Exit runserver when Daphne startup aborts

[lin-hongkuan]

Summary

• make the Django runserver command check Server.abort_start after Server.run() returns
• raise CommandError when Daphne aborts during startup, matching the standalone CLI behavior of exiting non-zero
• add a focused runserver startup test that simulates an abort from the server

Fixes #577

Tests

• python -m pytest tests\test_runserver_startup.py tests\test_cli.py -q
• python -m pytest -q tests --ignore=tests/test_packaging.py
• python -m black --check daphne\management\commands\runserver.py tests\test_runserver_startup.py
• python -m flake8 daphne\management\commands\runserver.py tests\test_runserver_startup.py
• git diff --check

Note: python -m pytest -q fails in my local editable-install environment only at tests/test_packaging.py::test_fd_endpoint_plugin_installed, because the Twisted plugin file is not present under the selected site-packages path. The runtime tests pass with that packaging-only test excluded.