Add DuplicateGadgetException for safety

[LeeTwentyThree]

Changes made in this pull request

• Add DuplicateGadgetException that is thrown when adding multiple gadgets of the same type without removing one beforehand.

Example of one exception in the log:

[Error  : Unity Log] DuplicateGadgetException: Cannot add Gadget of Type 'Nautilus.Assets.Gadgets.ScanningGadget' onto prefab of ClassID 'CopperClone' because a Gadget already exists on this prefab with the same type! Did you forget to call ICustomPrefab.RemoveGadget<TGadget>()?
Stack trace:
Nautilus.Assets.CustomPrefab.AddGadget[TGadget] (TGadget gadget) (at <2d6c53b33c57449cbcd48bf3b63cc3b1>:0)
Nautilus.Examples.CustomPrefabExamples.Awake () (at <136feb4898744685a66659a924b5e09c>:0)
UnityEngine.GameObject:AddComponent(Type)
BepInEx.Bootstrap.Chainloader:Start()
UnityEngine.Application:.cctor()
UWE.GameApplication:AppAwake()

[EldritchCarMaker]

Some more explicit mentions of it in the add gadget method could help, but overall seems fine