[Resolve #1484] Gracefully handle invalid parameters (#1485)

* [Resolve #1484] Gracefully handle invalid parameters

This adds logic to _cast_parameters to also handle the case that the
`parameters` dict itself is not a dict but some other invalid type.

sceptre/stack.py

@@ -305,6 +305,11 @@ def is_valid(value: Any) -> bool:
                 or isinstance(value, Resolver)
             )
 
+        if not isinstance(parameters, dict):
+            raise InvalidConfigFileError(
+                f"{self.name}: parameters must be a dictionary of key-value pairs, got {parameters}"
+            )
+
         casted_parameters = {k: cast_value(v) for k, v in parameters.items()}
 
         if not all(is_valid(value) for value in casted_parameters.values()):

tests/test_stack.py

@@ -202,6 +202,20 @@ def test_init__invalid_parameters_raise_invalid_config_file_error(self, paramete
                 parameters=parameters,
             )
 
+    @pytest.mark.parametrize(
+        "parameters",
+        [["this", "is", "a", "list"], "a_string"],
+    )
+    def test_init__invalid_parameters__parameters_a_list(self, parameters):
+        with pytest.raises(InvalidConfigFileError):
+            Stack(
+                name="stack_name",
+                project_code="project_code",
+                template_handler_config={"type": "file"},
+                region="region",
+                parameters=parameters,
+            )
+
     @pytest.mark.parametrize(
         "parameters",
         [