Consider augmenting _Scope to ensure we clear errors after each operation

We wrap all (or almost all) FFI code in `_Scope`, thus, it might make sense to review all usage of `_Scope` to see if we can augment `_Scope` such that that clear errors after an operation is done.

We should be handling errors all the places where they can happen, but as added safety it might be wise to clear errors after each operation and throw an error, if BoringCrypto has an error on the stack that we didn't expect to find there.