dirauth: Warn when failing to post during a vote

The only way to figure out that posting a vote or signatures to another
dirauth failed is by counting how many success messages there are on
notice level, and noticing that it is fewer than the number of
configured dirauths.

Closes #40910.

src/feature/dirclient/dirclient.c

@@ -101,7 +101,7 @@ dir_conn_purpose_to_string(int purpose)
     case DIR_PURPOSE_UPLOAD_DIR:
       return "server descriptor upload";
     case DIR_PURPOSE_UPLOAD_VOTE:
-      return "server vote upload";
+      return "consensus vote upload";
     case DIR_PURPOSE_UPLOAD_SIGNATURES:
       return "consensus signature upload";
     case DIR_PURPOSE_FETCH_SERVERDESC:
@@ -763,6 +763,11 @@ connection_dir_client_request_failed(dir_connection_t *conn)
              "directory server at %s; will retry",
              connection_describe_peer(TO_CONN(conn)));
     connection_dir_download_routerdesc_failed(conn);
+  } else if (conn->base_.purpose == DIR_PURPOSE_UPLOAD_VOTE ||
+             conn->base_.purpose == DIR_PURPOSE_UPLOAD_SIGNATURES) {
+    log_warn(LD_DIR, "Failed to post %s to %s.",
+             dir_conn_purpose_to_string(conn->base_.purpose),
+             connection_describe_peer(TO_CONN(conn)));
   }
 }