fix onload and onerror parsing error

[sehnryr]

regression error induced by 299b123 where serde default got removed from load_error attribute of ImageData.

pub struct ImageData {
-     #[cfg_attr(feature = "serialize", serde(default))]
-     pub load_error: bool,
+     inner: Box<dyn HasImageData>,
}

// ...

+ struct SerializedImageData {
+     load_error: bool,
+ }

Also, it seems like neither the load nor the error events are used in serializeEvent. This could be an issue when an error event goes unnoticed and the load_error attribute is still set to false as it is the default value for bool.

Something like this could do the trick but it might be out of scope for this PR.

if (event.type === "load") {
  extend({ load_error: false })
}
if (event.type === "error") {
  extend({ load_error: true })
}