throw ArgumentError on transitiveclosure!(::MetaGraph)

previously, this would not return an error but would not update the underlying graph structure.

Note: simply calling `transitiveclosure!(G.graph)` also fails as this does not update the metadata, e.g. `G.edge_data`.

Author: drbergman

src/metagraph.jl

@@ -248,3 +248,7 @@ This can be useful to interpret the results of methods inherited from `Graphs`.
 function label_for(meta_graph::MetaGraph, code::Integer)
     return meta_graph.vertex_labels[code]
 end
+
+function transitiveclosure!(meta_graph::MetaGraph)
+    throw(ArgumentError("transitiveclosure! not implemented for type MetaGraph"))
+end