Update current listing instead of appending.

Keep a list of elements seen by the web page and use that
to look up and update an already displaying element with
its new value. Create a display <li> for each new element
we encounter.
Monitor more elements.
Catch and log JSON.parse failures.

Sometimes we see multiple json stanzas in a single data
call, and JSON.parse() fails with a syntax error. Just
drop these until we can figure out a way to split them
properly.