Stream: fixed logging UDP upstream timeout.

Previously, when the first UDP response packet was not received from the
proxied server within proxy_timeout, no error message was logged before
switching to the next upstream.  Additionally, when one of succeeding response
packets was not received within the timeout, the timeout error had low severity
because it was logged as a client connection error as opposed to upstream
connection error.

src/stream/ngx_stream_proxy_module.c

diff --git a/src/stream/ngx_stream_proxy_module.c b/src/stream/ngx_stream_proxy_module.c
index 395e0f6..9d4b075 100644
--- a/src/stream/ngx_stream_proxy_module.c
+++ b/src/stream/ngx_stream_proxy_module.c
@@ -1331,13 +1331,17 @@
                     return;
                 }
 
+                ngx_connection_error(pc, NGX_ETIMEDOUT, "upstream timed out");
+
                 if (u->received == 0) {
                     ngx_stream_proxy_next_upstream(s);
                     return;
                 }
+
+            } else {
+                ngx_connection_error(c, NGX_ETIMEDOUT, "connection timed out");
             }
 
-            ngx_connection_error(c, NGX_ETIMEDOUT, "connection timed out");
             ngx_stream_proxy_finalize(s, NGX_STREAM_OK);
             return;
         }