G30 improvements

Consider flagging an error when G30 P# Z-9999 is sent with no X and Y co-ordinates.

Also G30 implementations with and without probe offsets can be confusing so either documentation clarification required or simplification on how it should work (provide errors on all invalid/unconsidered uses)

https://forum.duet3d.com/topic/25513/g30-behaving-strangely-random-moving-between-probe/12