drag_planet.php: handle target sector has planet
drag_planet.php: handle target sector has planet

Fail gracefully if the target sector already has a planet. Previously,
it would throw an exception and ajax updates would break until the page
was refreshed.

Note that we could add an ajaxified warning message in this case, but
the behavior should be sufficiently self-explanatory.

hemberger/smr,hemberger/smr,hemberger/smr,smrealms/smr,smrealms/smr,hemberger/smr,smrealms/smr