Make sure all exceptions from cppmm are handled

[scott-wilson]

We ran into an issue yesterday where an error wasn't handled in the safe API, causing a segfault. In order to avoid those for now, we should make sure that all functions from cppmm are handled. Even if it is just sys::Imf_func(...).into_result().unwrap().

The PR template should be updated to include that as a requirement for accepting a PR as well.