Audit FFI methods return values

[sanket1729]

The rust type system can provide some guarantees w.r.t calling arguments of FFI functions

1. We can cleanly document the cases in which the FFI method can return an Error (supplying a negation of tweak in an add_tweak method), or safely unwrap them if our type system guarantees it.
2. I also some while reviewing Add convenience methods for keys #430 that were addressed in the same PR.

[sanket1729]

I can do this in a couple of weeks. But if somebody wants to do it, it's up for grabs.