[mod] don't dump traceback of SearxEngineResponseException on init

When initing engines a "SearxEngineResponseException" is logged very verbose,
including full traceback information:

    ERROR:searx.engines:yggtorrent engine: Fail to initialize
    Traceback (most recent call last):
      File "share/searx/searx/engines/__init__.py", line 293, in engine_init
        init_fn(get_engine_from_settings(engine_name))
      File "share/searx/searx/engines/yggtorrent.py", line 42, in init
        resp = http_get(url, allow_redirects=False)
      File "share/searx/searx/poolrequests.py", line 197, in get
        return request('get', url, **kwargs)
      File "share/searx/searx/poolrequests.py", line 190, in request
        raise_for_httperror(response)
      File "share/searx/searx/raise_for_httperror.py", line 60, in raise_for_httperror
        raise_for_captcha(resp)
      File "share/searx/searx/raise_for_httperror.py", line 43, in raise_for_captcha
        raise_for_cloudflare_captcha(resp)
      File "share/searx/searx/raise_for_httperror.py", line 30, in raise_for_cloudflare_captcha
        raise SearxEngineCaptchaException(message='Cloudflare CAPTCHA', suspended_time=3600 * 24 * 15)
    searx.exceptions.SearxEngineCaptchaException: Cloudflare CAPTCHA, suspended_time=1296000

For SearxEngineResponseException this is not needed.  Those types of exceptions
can be a normal use case.  E.g. for CAPTCHA errors like shown in the example
above. It should be enough to log a warning for such issues:

    WARNING:searx.engines:yggtorrent engine: Fail to initialize // Cloudflare CAPTCHA, suspended_time=1296000

closes: #2612

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>