Reject a non-string execute command with a clear installer error

tests/_test_installer.py

@@ -78,6 +78,12 @@ class TestScriptInterpreter(TestCase):
             interpreter._map_command({"_substitute": "foo"})
         self.assertEqual(ex.exception.message, 'The command "substitute" does not exist.')
 
+    def test_execute_rejects_a_command_that_is_not_a_string(self):
+        interpreter = MockInterpreter(TEST_INSTALLER, None)
+        with self.assertRaises(ScriptingError) as ex:
+            interpreter.execute({"command": [["pwd"]]})
+        self.assertEqual(ex.exception.message, "The command parameter of the execute command must be a string")
+
     def test_prepare_game_files_resolves_moddb_url(self):
         moddb_url = "https://www.moddb.com/downloads/test-file"
         mirror_url = "https://www.moddb.com/downloads/mirror/test-file"