sd-bus: add custom return code when $XDG_RUNTIME_DIR is not set

We would return ENOENT, which is extremely confusing. Strace is not helpful because
no *file* is actually missing. So let's add some logs at debug level and also use
a custom return code. Let all user-facing utilities print a custom error message
in that case.

src/systemctl/systemctl-util.c

@@ -52,7 +52,7 @@ int acquire_bus(BusFocus focus, sd_bus **ret) {
                 else
                         r = bus_connect_transport(arg_transport, arg_host, user, &buses[focus]);
                 if (r < 0)
-                        return log_error_errno(r, "Failed to connect to bus: %m");
+                        return bus_log_connect_error(r);
 
                 (void) sd_bus_set_allow_interactive_authorization(buses[focus], arg_ask_password);
         }