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To be honest, it kind of seems odd to return an RSS feed in case of an error, since errors can't really be expressed in that format. At the very least if an rss feed is returned in error conditions it should be a non-200 status code imho.
Thank you for the comment, I agree with what you said, that means the custom printer needs to be altered to some other printers on error since the custom printer requires some additional parameters which exception does not provide, that doesn't seem to be an easy fix to me as the API can be executed in a mode with exception not being caught and there looks quite a lot of dependencies in ApiMain.php, However, there may be an easy fix to this which I don't see so I am open to any feedbacks, :)
Change was copied to Gerrit as https://gerrit.wikimedia.org/r/3328
Follow up there :)