Operations to not throw Exceptions

[ht-gong]

Currently add/delete_node/edge throws an exception if a WT error other than WT_Rollback (or sometimes WT_notfound) is encountered -- change all of these to return an error code instead of throws

[artorhem]

Throwing exceptions HAS to be avoided. Need to go through the code to see where we really do need to throw an exception and where a simple error log will suffice.

for example: in get_out_node_id(), if there is no valid out neighborhood, we throw an exception, which is absolutely wrong.