logging: respect error_log in nginx.conf server blocks

This makes messages written via the NgxMessageHandler from the
NgxServerContext be written with the nginx error_log configuration
specified at the server{} level, if specified.

Fixes https://github.com/pagespeed/ngx_pagespeed/issues/447
Merge pull request #448 from pagespeed/oschaaf-log-vhost-specific

logging: respect error_log in nginx.conf server blocks