MissingEdgeError for overlap embeddings

[stefanhannie]

Current Problem When working with overlap embeddings (embeddings where overlapping chains are permitted), MissingEdgeErrors are being thrown when edges are not missing.

Proposed Solution Rework dwave.embedding.diagnose_embedding(...):

• Change label = {} so that the values are are collections of vertices from the source graph (not just singletons)
• Change the graph yielded to contain all edges between overlapped chains.

Alternatives Considered One can write a function to test the validity of each MissingEdgeErrors if they are working with overlapped embeddings.

Additional context @boothby