[Bug] .createConstructionSite should not place multiple construction sites at the same position

[antonnyst]

.createConstructionSite should return ERR_INVALID_TARGET instead of OK if a construction site already exists at the specified location.
To reproduce run for example:
new RoomPosition(25,25,"W8N3").createConstructionSite(STRUCTURE_ROAD);
new RoomPosition(25,25,"W8N3").createConstructionSite(STRUCTURE_RAMPART);