@roger,
We had a similar experience, when euthanizing our kitty after two cancer surgeries. The muscle relaxant caused nausea and vomiting, but not something I'd call convulsions.
If we'd known about the nausea, we would have known not to give her treats before the procedure. The vet was the nicest person imaginable, and even made a house call for the procedure.
Believe me, these folks don't want animals to suffer.