[fix] startpage engine: fetch CAPTCHA & issues related to PR-695

In case of CAPTCHA raise a SearxEngineCaptchaException and suspend for 7 days.
When get_sc_code() fails raise a SearxEngineResponseException and suspend for 7
days.

[1] https://github.com/searxng/searxng/pull/695



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