Improve the message of validate_pluggable_transports_config().

         pluggable transports. We should warn the user in such case,

         since it's probably misconfiguration. */

      if (!transport_get_by_name(b->transport_name))

        log_warn(LD_CONFIG, "You have a Bridge line using the %s "

                 "pluggable transport, but there doesn't seem to be a "

                 "corresponding ClientTransportPlugin line.",

                 b->transport_name);

        log_warn(LD_CONFIG, "We can't find a pluggable transport proxy "

                 "that supports '%s' for bridge '%s:%u'. This can happen "

                 "if you haven't provided a ClientTransportPlugin line, or "

                 "if your pluggable transport proxy stopped working.",

                 b->transport_name, fmt_addr(&b->addr), b->port);

    } SMARTLIST_FOREACH_END(b);

    return 0;