Infinite Recursion error on graph that is not a tree

[bdemchak]

For many graphs, it's hard for a user to easily see whether the graph is a tree. If the user does not select a node or the node is not the parent of a tree, cytoscape-d3 throws an exception (Infinite recursion StackOverflowError) and displays the error to the user. Much better to give diagnostic error messages that tell the user to either select a node or to make the graph a tree.