better exceptions for testing.html
better exceptions for testing.html

Now, exceptions are throw properly as angry red text
with a stack trace rather than black text with a
mediocre stack trace.

Also, errors don't stop all rendering: other tests after
the failing one will continue to render.

iobeam/plottable,onaio/plottable,jacqt/plottable,gdseller/plottable,gdseller/plottable,alyssaq/plottable,danmane/plottable,jacqt/plottable,softwords/plottable,softwords/plottable,gdseller/plottable,iobeam/plottable,alyssaq/plottable,palantir/plottable,palantir/plottable,RobertoMalatesta/plottable,alyssaq/plottable,iobeam/plottable,softwords/plottable,onaio/plottable,danmane/plottable,jacqt/plottable,NextTuesday/plottable,palantir/plottable,RobertoMalatesta/plottable,RobertoMalatesta/plottable,NextTuesday/plottable,palantir/plottable,onaio/plottable,danmane/plottable,NextTuesday/plottable