Prefer to use safer self.is_connected() as it checks interface
is not None.
Using common code gives small observable changes in behaviour:
- slightly different print_error() messages
- network restarts now enter status 'connecting' which they
didn't before, which seems correct
- status 'connecting' is done with set_status() rather than
simply assigning the status, which seems more correct. Now
that the response_queue is available in the constructor this
works; it used to fail with 'response_queue is not defined'
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