I am sorry to read of your loss.
We had a little cat, April Dancer, that my wife rescued from a parking lot on the USAF Base where we were stationed at the time.
http://www.medhelp.org/user_photos/show/94337?personal_page_id=40606 (link to a picture of her)
Dancer was our house gremlin, always looking out for us and the other three kitties (Victoria, Cammi and Cerrie).
She, too, passed under a vet's care; anaphylactic shock from a mild sedative during a routine procedure. I'm a veteran and saw a little bit of combat, did well according to my older, more experienced comrades...never cried so hard as when that little critter died while we were trying to do right by her.
Sometimes you do everything right, but you still get a wrong result.
Percy did have a richness in his life, you and your love for him; something a lot of kitties do not get in their lives.
Take care - Pip