This condition is signaled when an edge cannot be found in a graph.
This is the root condition for errors that occur while running code in CL-Graph.
The `graph` of which this edge is a part.Initargs::graph; Reader:graph.
`Vertex-1` is one of the two vertexes that an edge connects. In a directed-edge, `vertex-1` is also the `source-edge`.Initargs::vertex-1; Reader:vertex-1.
`Vertex-2` is one of the two vertexes that an edge connects. In a directed edge, `vertex-2` is also the `target-vertex`.Initargs::vertex-2; Reader:vertex-2.