httpservice: move sys.exit() out of serve_forever()
This is a simple refactoring to show the callers of the method, so
it's easier to reason about the impact of removing the `sys.exit()`
calls in subsequent patches.
Differential Revision: https://phab.mercurial-scm.org/D9270
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true