hgweb: don't call sys.exit() in httpservice.run() If I'm reading the code correctly, `mercurial.server.createservice()` can return an hgweb service or one of three types of command server services. The caller then calls `mercurial.server.runservice()`, passing it the returned service's run method. Only the hgweb service was calling `sys.exit()`. It has been that way since 8d44649df03b (refactor ssh server., 2006-06-04). That commit message doesn't provide any explanation. Let's clean up and have the code follow the usual return path into the `dispatch` module. After this patch, there should be no remaining places left where we call `sys.exit()` except for valid uses in the `dispatch` and `worker` modules. Differential Revision: https://phab.mercurial-scm.org/D9272