add listen address in error message

src/http/ngx_http.c

diff --git a/src/http/ngx_http.c b/src/http/ngx_http.c
index 6258380..15b53fe 100644
--- a/src/http/ngx_http.c
+++ b/src/http/ngx_http.c
@@ -1215,7 +1215,7 @@
 
             if (addr[i].opt.default_server) {
                 ngx_conf_log_error(NGX_LOG_EMERG, cf, 0,
-                                   "the duplicate default server");
+                        "a duplicate default server for %s", addr[i].opt.addr);
                 return NGX_ERROR;
             }