Improve the diagnostic messages in plugins

This looks weird, since we already pointed this is a userland issue, maybe it should not be `UNHANDLEABLE` anymore?

_Originally posted by @IWANABETHATGUY in https://github.com/rolldown/rolldown/pull/4731#discussion_r2113456648_
            