avoid exception in sliverstatus when get_slice_cred failed
avoid exception in sliverstatus when get_slice_cred failed

tcmitchell/geni-tools,plantigrade/geni-tools,ahelsing/geni-tools,GENI-NSF/gram,ahelsing/geni-tools,plantigrade/geni-tools,tcmitchell/geni-tools,GENI-NSF/gram,GENI-NSF/gram