http 常见状态码

X-Crawlera-Error	Response Code	Error Message
bad_session_id	400	Incorrect session ID
user_session_limit	400	Session limit exceeded
bad_auth	401	Unauthorized mashape request
bad_auth	407
too_many_conns	429	Too many connections*
header_auth	470	Unauthorized Crawlera header
	500	Unexpected error
nxdomain	502	Error looking up domain
econnrefused	502	Connection refused
econnreset	502	Connection reset
socket_closed_remotely	502	Server closed socket connection
send_failed	502	Send failed
noslaves	503	No available proxies
slavebanned	503	Website crawl ban
serverbusy	503	Server busy: too many outstanding requests
timeout	504	Timeout from upstream server
msgtimeout	504	Timeout processing HTTP stream
domain_forbidden	523	Domain forbidden. Please contact support@scrapinghub.com
bad_header	540	Bad header value for <some_header>