Return string indicating the error encountered on failure.

def status(self):
        """Return string indicating the error encountered on failure."""
        self._check_valid()
        if self._ret_val == swiglpk.GLP_ENOPFS:
            return 'No primal feasible solution'
        elif self._ret_val == swiglpk.GLP_ENODFS:
            return 'No dual feasible solution'
        return str(swiglpk.glp_get_status(self._problem._p))