change assert is_controller_okay to log.warn and count the error

Created by: juga0

I think that we should not get that exception and instead log the error and count one more circuit error (if it is not already being count, i've not checked). @pastly: if it easy for you, would you do it?, otherwise i'll check:

[2018-06-15 16:58:51,788] [sbws.core.scanner] [ERROR] Unhandled exception in worker thread
Traceback (most recent call last):
  File "/path/simple-bw-scanner/sbws/core/scanner.py", line 257, in dispatch_worker_thread
    return measure_relay(*a, **kw)
  File "/path/simple-bw-scanner/sbws/core/scanner.py", line 170, in measure_relay
    cb.controller, conf.getfloat('general', 'http_timeout'))
  File "/path/simple-bw-scanner/sbws/util/requests.py", line 7, in make_session
    socks_info = stem_utils.get_socks_info(controller)  
  File "/path/simple-bw-scanner/sbws/util/stem.py", line 197, in get_socks_info
    assert is_controller_okay(controller)
AssertionError

Also:

Traceback (most recent call last):
  File "/path/.virtualenvs/simplebwscanner3/bin/sbws", line 11, in <module>
    load_entry_point('sbws', 'console_scripts', 'sbws')()
  File "/path/simple-bw-scanner/sbws/sbws.py", line 51, in main
    exit(comm['f'](*comm['a'], **comm['kw']))
  File "/path/simple-bw-scanner/sbws/core/scanner.py", line 402, in main
    run_speedtest(args, conf)
  File "/path/simple-bw-scanner/sbws/core/scanner.py", line 362, in run_speedtest
    for target in rp.best_priority():
  File "/path/simple-bw-scanner/sbws/lib/relayprioritizer.py", line 49, in best_priority
    relays = copy.deepcopy(self.relay_list.relays)
  File "/path/simple-bw-scanner/sbws/lib/relaylist.py", line 132, in relays
    self._refresh()
  File "/path/simple-bw-scanner/sbws/lib/relaylist.py", line 175, in _refresh
    self._relays = self._init_relays()
  File "/path/simple-bw-scanner/sbws/lib/relaylist.py", line 169, in _init_relays
    assert stem_utils.is_controller_okay(c)