I hadn't picked up on this either
1. how they can fail him
2. and how that would make their relationship still good

I will have to watch that scene again but going by the dialog, it doesn't make any sense
Then again, Michael's been careless, saying stupid things....

I too reckon Michael was not expecting Pentangeli to come to any harm because he needs the muscle and if he did, no doubt Michael can't afford to let his man's murder go unavenged