Improve Rust error handling

[tholop]

Handle errors in the Rust code instead of using assert and assert_eq macros.

[dimasquest]

Done some rust before, can I try this one?

[tholop]

That would be helpful, thanks! There are some good candidates in https://github.com/OpenMined/sycret/blob/master/src/eq.rs and le.rs in particular. Let me know if you have any questions!