When you add too much butter, it adds increased amounts of fat to your mixture/batter, meaning that the cake will be very greasy. Due to the ratio change in the recipe, this will also prevent the cake from rising like it should do and may alter the taste too.
The good thing is you are aware of what you did, and you'll know not to make that mistake again. What I always do when using any recipe is lay the ingredients out on the work surface in the order I need to use them. That way, nothing gets missed out, and nothing gets added in. You can always double-check your quantities at each stage to ensure you use the correct amounts.
I do hope you manage to have a bit of cake. You always need to taste test. That's the best part.