Search graph for an edge whose vertexes match
edge. This means that vertex-1 of the edge in the graph must
match vertex-1 of edge and so forth. Wil signal an error of type
graph-edge-not-found-error unless error-if-not-found? is
nil. [?? Unused. Remove?]
| find-edge < cl-graph:graph-container > < cl-graph:graph-container-edge > |