Add checks for add_node to protect against nodes with non-existent dependency

Need to add guardrails and throw error if node is added with a non-existent dependency.

Currently, this will cause the node to simply be ignored.

Start by locating the add_node entry point and tracing how dependencies are checked or stored. Reproduce the case where a node names a non-existent dependency, then determine the existing error-handling and test patterns; done means the invalid dependency is rejected with an error rather than silently ignored.

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