Ensure to always use the project's nginx error_log

Problem

Onionspray is using the NGINX error log from the installation location, and not from the project log folder. In the following example, an error message when using the OpenResty upstream Debian package:

user@onionspray:/srv/shared$ ./onionspray start -a
:::: start tpo ::::
nginx: [alert] could not open error log file: open() "/usr/local/openresty/nginx/logs/error.log" failed (13: Permission denied)

Tasks

• Ensure to always use the project's nginx error_log.

Time estimation

• Complexity: very small (0.5 day)
• Uncertainty: low (x1.1)
• Reference (adapted)