What is the expected enhancement?

Currently, setting source to non present indices in bfs_search will panic.

Running

import rustworkx as rx
from rustworkx.visit import BFSVisitor

graph = rx.PyGraph()
rx.bfs_search(graph, [100], BFSVisitor())

raises:

Traceback (most recent call last):
pyo3_runtime.PanicException: put at index 100 exceeds fixedbitset size 0

I suggest:

1. raising an IndexError instead.
2. In rustworkx-core either returning an Error or documenting the panic behavior
3. If needed do the same for other traversals (dfs, dijkstra).