Improve error handling

[kianenigma]

Currently, there are two panic!s and one except() in the Rust code. They should be removed. Consequently, the entire interface could -> should be replaced with returning Result<T, JsVal>.

https://rustwasm.github.io/wasm-bindgen/reference/attributes/on-js-imports/catch.html

[kianenigma]

will be fixed in when #4 is merged.