False Creation of Children Nodes

[adrianmross]

https://github.com/adrianmross/standalone-scp/blob/464219e350e8be047590915242ce1099d86ace6e/parser.cpp#L142-L144

When accessing a mapping like curr_node = name_to_node[node_name]; if the key node_name doesn't yet exist in the mapping name_to_node it will create a new entry to the mapping. We don't want this, so should throw an error and handle output.