Replace `self.server.error_log()` with `self.error_log()` in `HTTPServer._serve_unservicable()`

[webknjaz]

There is no self.server within HTTPServer.

Originally posted by @martin-vi in #745 (comment)

cc @itamarst @pythonspeed

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Solving this involves both doing what $sbj says and writing a test that covers the respective line.