mod_proxy: Handle ap_proxy_buckets_lifetime_transform() errors.

mod_proxy: Handle ap_proxy_buckets_lifetime_transform() errors.

* modules/proxy/mod_proxy.h,modules/proxy/proxy_util.c:
  Add ap_proxy_fill_error_brigade() to factorize proxy error handling
  on the client connection side.

* modules/proxy/mod_proxy_{http,ajp,uwsgi}.c:
  Use ap_proxy_fill_error_brigade() where needed, including when an
  empty brigade is returned on the backend side or when calling
  ap_proxy_buckets_lifetime_transform fails.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1893595 13f79535-47bb-0310-9956-ffa450edef68