more error checks on the slice credential we are about to return
more error checks on the slice credential we are about to return

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