Added AbbreviatedException.__str__ to help pure Python users
Added AbbreviatedException.__str__ to help pure Python users

vascotenner/holoviews,ioam/holoviews,vascotenner/holoviews,basnijholt/holoviews,ioam/holoviews,basnijholt/holoviews,basnijholt/holoviews,vascotenner/holoviews,ioam/holoviews