Throw error if user tries to specify an invalid failure group

[adammoody]

Both XOR and PARTNER require at least two processes in each redundancy group. Currently the code segfaults if the user gives a failure group such that some process ends up in a redundancy group of size 1. To be helpful, we should print and return an error instead. With SCR, we would automatically fallback to SINGLE in such cases. That could be another option.