Backend notifies frontend about errors; backend refuses to count too many items except at priming.

- The backend notifies the frontend if there was an error handling any individual view.
- The backend refuses to do counts over more than a (user-settable) number of items, and sends an error to the frontend if that happens. This is disabled for priming, where all counts are done fully.
Shorter timeline event count axis label.
Changed frontend readme to reflect change from closure to uglifyjs.