Proxy: use an appropriate error on memory allocation failure.

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 d7675d4..2409c7e 100644
--- a/src/http/modules/ngx_http_proxy_module.c
+++ b/src/http/modules/ngx_http_proxy_module.c
@@ -812,7 +812,7 @@
 
     ctx = ngx_pcalloc(r->pool, sizeof(ngx_http_proxy_ctx_t));
     if (ctx == NULL) {
-        return NGX_ERROR;
+        return NGX_HTTP_INTERNAL_SERVER_ERROR;
     }
 
     ngx_http_set_ctx(r, ctx, ngx_http_proxy_module);