Test errors during server life.

* bzrlib/tests/test_test_server.py:
(TCPClient.disconnect): Be more robust on errors.
(TestTestingServerInAThread.get_server): Add some optional
parameters.
(TestTestingServerInAThread.test_server_fails_to_start)
(TestTestingServerInAThread.test_server_fails_while_serving_or_stoping):
Test exception catching in during the significant steps.

* bzrlib/tests/test_server.py:
(TestingTCPServer.handle_error): Override the default behavior, we
want the exception to be raised.
(TestingTCPServerInAThread.start_server): Once the server is
started, we need another event to sync with the main thread.
(TestingTCPServerInAThread.stop_server): Don't forget to join the
thread !