Upstream: NGX_HTTP_GATEWAY_TIME_OUT after upstream timeouts.

Upstream: NGX_HTTP_GATEWAY_TIME_OUT after upstream timeouts.

There is no real difference from previously used 0 as NGX_HTTP_* will
become 0 in ngx_http_upstream_finalize_request(), but the change
preserves information about a timeout a bit longer.  Previous use of
ETIMEDOUT in one place was just wrong.

Note well that with cacheable responses there will be a difference
(code in ngx_http_upstream_finalize_request() will store the error
in cache), though this change doesn't touch cacheable case.