called when self.runtest() raises an exception.

def repr_failure(self, excinfo):
        """ called when self.runtest() raises an exception. """
        exc = excinfo.value
        cc = self.colors
        if isinstance(exc, NbCellError):
            msg_items = [
                cc.FAIL + "Notebook cell execution failed" + cc.ENDC]
            formatstring = (
                cc.OKBLUE + "Cell %d: %s\n\n" +
                "Input:\n" + cc.ENDC + "%s\n")
            msg_items.append(formatstring % (
                exc.cell_num,
                str(exc),
                exc.source
            ))
            if exc.inner_traceback:
                msg_items.append((
                    cc.OKBLUE + "Traceback:" + cc.ENDC + "\n%s\n") %
                    exc.inner_traceback)
            return "\n".join(msg_items)
        else:
            return "pytest plugin exception: %s" % str(exc)