Adds condition to require exactly one exit point

Note that Legacy didn't have a check because the actual exits were calculated based on the bottom opening blocks in order to center a leaving player. A warning would instead be displayed upon teleportation when a player tried to teleport to a Stargate with no exit.

src/main/java/net/knarcraft/stargate/portal/property/gate/GateHandler.java

@@ -211,6 +211,10 @@ public class GateHandler {
             return false;
         }
         
+        if (gate.getLayout().getExit() == null) {
+            Stargate.logSevere(String.format(failString, "Gates must have one specified exit point"));
+        }
+
         if (checkMaterialPredicateFail(gate.getPortalButtonMaterials(), MaterialHelper::isButtonCompatible)) {
             Stargate.logSevere(String.format(failString, "Gate button must be a type of button."));
             return false;