#[wasm_bindgen(setter, js_name = ...)]
Previously we'd require the explicit `js_name` to *also* start with `set_`, but when explicitly specified it shouldn't be mangled at all! Closes #584
infer_setter_property
If the setter doesn't start with `set_*` then we currently panic, but panicking is bad! Instead let's thread through structured errors to make sure they make their way to the top
