Fix for "element-path" not validated before use

_project.py: Added node_validate before yaml is used

buildstream/_project.py

@@ -402,6 +402,17 @@ class Project():
                 "Project requested format version {}, but BuildStream {}.{} only supports up until format version {}"
                 .format(format_version, major, minor, BST_FORMAT_VERSION))
 
+        _yaml.node_validate(pre_config_node, [
+            'format-version',
+            'element-path', 'variables',
+            'environment', 'environment-nocache',
+            'split-rules', 'elements', 'plugins',
+            'aliases', 'name',
+            'artifacts', 'options',
+            'fail-on-overlap', 'shell', 'fatal-warnings',
+            'ref-storage', 'sandbox', 'mirrors', 'remote-execution'
+        ])
+
         # FIXME:
         #
         #   Performing this check manually in the absense