zs computation method return -1 when error occur, whereas "-1" is a valid zs value

Replace error code value to "-1000"

changed milestone to %1.6.4

created merge request !51 (merged) to address this issue

mentioned in merge request !51 (merged)

created merge request !52 (merged) to address this issue

mentioned in merge request !52 (merged)

mentioned in issue #63 (closed)

closed