Fail service call if config file cannot be read

This includes the case where there is an I/O error or it is e.g. a
symlink to a nonexistent file.

Fixes: QubesOS/qubes-issues#9100

Author: DemiMarie

libqrexec/exec.c

@@ -285,7 +285,7 @@ static int load_service_config_raw(struct qrexec_parsed_command *cmd,
     if (ret == -1)
         ret = find_file(config_path, cmd->service_name,
                         config_full_path, sizeof(config_full_path), NULL);
-    if (ret < 0)
+    if (ret == -1)
         return 0;
     return qubes_toml_config_parse(config_full_path, &cmd->wait_for_session, user,
                                    &cmd->send_service_descriptor);

qrexec/tests/socket/agent.py

@@ -541,7 +541,6 @@ def test_exec_service_with_invalid_config_4(self):
     def test_exec_service_with_invalid_config_5(self):
         self.exec_service_with_invalid_config("wait-for-session\n")
 
-    @unittest.expectedFailure
     def test_exec_service_with_invalid_config_6(self):
         self.exec_service_with_invalid_config(None)