tell user which node is being sampled when checkLogProb warning invoked

[paciorek]

This augments the checkLogProb warning to say what target node is being sampled when it is invoked.
If there is more than one node, I'm not sure how ugly the warning would look in terms of the multiple node names being pasted together.

This is in partial response to NCT issue 572.

Ideally we would also:

• (perhaps) say what nodes are being calculated that cause the issue (there could be many of them, making the message hard to format/too long)
• allow the user to opt out of the warning entirely
• only print 1 or a small number of warnings for any given target node for which this occurs rather than printing every time it occurs
• refresh the progress bar so the warning does not make it hard to see where the progress bar stands

I'm not sure how to easily (or at all) do those things (in part because this is occurring in nf code rather than in R).

At the moment, I am thinking this is a worthwhile enhancement without getting deeper into improving this. But it's worth some discussion.