Upload: lack of documentation (or support) for passing errors from `:external` 2-arity function

[DaTrader]

Environment

• Phoenix version (mix deps): 0.20.17

Actual behavior

Upload config: external, autoload

I couldn't find any documentation on what the :external 2-arity function is supposed to return in the meta map in { :error, meta, socket} for the error to end up in the UploadConfig structure i.e. to become retrievable with Phoenix.Component.upload_errors.

Has this feature been developed yet or is planned to be developed or it's completely up to developers to handle such errors separately from the predefined upload ones?

Expected behavior

There should be a word in the docs on how to deal with those errors now and in the future.

PS. It would also be great if the value provided to entry.error/1 in a JS uploader was actually propagated along or instead of the external_client_failure atom.

Thanks