The death rate of pets under anesthesia is higher than in humans due to pets not having dedicated anesthetist to pick up subtle signs the pet is having problems. They could possibly not have a dedicated anesthetist that could monitor the slight signs the pet could have cardiac and respiratory arrest. Ask who was monitoring the anesthesia...
https://www.huffingtonpost.com/donna...b_1968950.html
This article has some questions that could help you ask the veterinarian to help you understand more what happened.
Could be possible an anesthesia allergy. These are the symptoms. Maybe ask your vet if she had some of these symptoms put in this article.
https://wagwalking.com/condition/anesthesia-allergies
Sorry for your loss. I hope you find some peace in knowing you did your best in loving and taking care of Ernie. Sometimes things are out of our hands.