'''Syntax:'''	'''proxy_next_upstream''' error | timeout | invalid_header | http_500 | http_502 | http_503 | http_504 | http_404 | off ...
'''Default:'''	''error timeout''
'''Context:'''	http server location
'''Reference:'''	[http://nginx.org/en/docs/http/ngx_http_proxy_module.html#proxy_next_upstream proxy_next_upstream]