update r3945 with more descriptive error message

src/http/modules/ngx_http_proxy_module.c

diff --git a/src/http/modules/ngx_http_proxy_module.c b/src/http/modules/ngx_http_proxy_module.c
index 214fe6a..147422a 100644
--- a/src/http/modules/ngx_http_proxy_module.c
+++ b/src/http/modules/ngx_http_proxy_module.c
@@ -1707,6 +1707,8 @@
     conf->headers_hash_max_size = NGX_CONF_UNSET_UINT;
     conf->headers_hash_bucket_size = NGX_CONF_UNSET_UINT;
 
+    ngx_str_set(&conf->upstream.module, "proxy");
+
     return conf;
 }