Objects should allow for error reporting in to_i64(), to_f64(), to_usize() and to_bigint()

[phorward]

In case an object cannot be converted to to_i64(), to_f64(), to_usize() or to_bigint(), it should be possible to provide an error message.

Todos:

• [x] Implement as Result<T, String>
• [ ] Implement as Result<T, Error>