'return the exception as a string
when \'tryshort\' resolves to True, and the exception is a
py.code._AssertionError, only the actual exception part of
the exception representation is returned (so \'AssertionError: \' is
removed from the beginning)'

def exconly(self, tryshort=False):

    lines = format_exception_only(self.type, self.value)
    text = ''.join(lines)
    text = text.rstrip()
    if tryshort:
        if text.startswith(self._striptext):
            text = text[len(self._striptext):]
    return text
