roles/nginx: Increase error log level to warn

This will show warnings in the error log, as well as more severe
log messages.

See: http://nginx.org/en/docs/ngx_core_module.html#error_log

roles/nginx/files/nginx.conf

@@ -9,7 +9,7 @@ worker_processes  auto;
 
 # Log errors and warnings to this file
 # This is only used when you don't override it on a server{} level
-error_log  /var/log/nginx/error.log error;
+error_log  /var/log/nginx/error.log warn;
 
 # The file storing the process ID of the main process
 pid        /var/run/nginx.pid;