Handle lag better in drill

If a mining drill is apparently applied to a non-pointable node, do
nothing rather than drilling as normal.  This situation usually arises
from lag, where the news of a node having been drilled didn't reach the
user quickly enough and the user thereby applied the drill twice to the
same node.  The second drill attempt would formerly consume charge and
then find that all the nodes it wanted to dig had already been removed.

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