show traceback in widget handlers
show traceback in widget handlers

rather than logging

closes #5064

cornhundred/ipywidgets,ipython/ipywidgets,jupyter-widgets/ipywidgets,ipython/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,cornhundred/ipywidgets,jupyter-widgets/ipywidgets,cornhundred/ipywidgets,jupyter-widgets/ipywidgets,SylvainCorlay/ipywidgets,cornhundred/ipywidgets,ipython/ipywidgets,cornhundred/ipywidgets,ipython/ipywidgets,SylvainCorlay/ipywidgets