PyDiGraph.add_edge() and PyGraph.add_edge panic if an endpoint doesn't exist

[mtreinish]

Information

• rustworkx version: 0.12.1
• Python version: 3.11
• Rust version: 1.68.2
• Operating system: Linux

What is the current behavior?

If you try to add an edge to a graph object with endpoints that aren't present in the graph petgraph panics and that causes an catchable exception to be raised from rustworkx without a user actionable message.
For example:

pyo3_runtime.PanicException: StableGraph::add_edge: node index 3 is not a node in the graph

What is the expected behavior?

I think it would be better to raise an IndexError or a custom rustworkx exception class. The issue is a PanicException isn't something users can catch easily (as it doesn't inherit from Exception) and is supposed to be treated as irrecoverable. Also the error message isn't something a python end user will understand. We should do checks that the indices are present in the graph before calling petgraph and return an appropriate error instead.

Steps to reproduce the problem

g = rx.PyDiGraph()
g.add_edge(2, 3)

or

g = rx.PyGraph()
g.add_edge(2, 3)