Throw error if added edges are invalid

This is the case if both the origin and the target of the edge point to
the same node or if an edge already exists between two nodes.

Graph.lua

@@ -31,6 +31,13 @@ function Graph.new()
     end
 
     function self:addEdge( origin, target )
+        for _, edge in pairs(edges) do
+            if edge.origin == origin and edge.target == target then
+                error "Trying to connect nodes which are already connected.";
+            end
+        end
+
+        assert(origin ~= target, "Tried to connect a node with itself.");
         edges[edgeIDs] = Edge.new( edgeIDs, origin, target );
         edgeIDs = edgeIDs + 1;
     end