chore: return expected errors without throwing exceptions

[ashneverdawn]

When a Result<> is returned from a function exposed to js via #[wasm_bindgen], the error is returned as an Exception.

Since panics also cause Exceptions, there is no way to tell if an Exception was caused by an expected error or a panic.

This matters because panics can potentially result in a corrupted wasm instance and should be handled by re-initializing a new wasm instance. Expected errors on the other hand (for example, the result of invalid user input) don't require re-initializing the entire wasm as they are just part of regular business logic.

For this reason, we would rather return expected errors as part of the Ok of this Result, instead of the Error.

So something like this:

#[wasm_bindgen]
pub fn my_func() -> Result<MyOkType, MyErrorType>

Would become something like this:

#[wasm_bindgen]
pub fn my_func() -> Result<Result<MyOkType, MyErrorType>, JsError>