We now give a helpful error message if the Thing is not found,
rather than returning null. There's an argument to return
`undefined` instead of raising an error, returning undefined
is the default behaviour.
I had intended to store a ConsumedThing for each Thing, in
a module in the store. Until that is possible (will require some
more attention to dependency management) I will just store
the TDs instead.
This commit includes a lot of the code I wrote for node-wot use,
but without any dependence on node-wot.
