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
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