Fail early if the service config cannot be found

If finding the service config fails with an I/O error, bail out
promptly.

Author: DemiMarie

libqrexec/exec.c

@@ -327,6 +327,8 @@ static int load_service_config_raw(struct qrexec_parsed_command *cmd,
                         config_full_path, sizeof(config_full_path), NULL);
     if (ret == -1)
         return 0;
+    if (ret != 0)
+        return ret;
     return qubes_toml_config_parse(config_full_path, &cmd->wait_for_session, user,
                                    &cmd->send_service_descriptor,
                                    &cmd->exit_on_stdout_eof,