MRF rejections should not log as errors

There was still one error left "Could not decode user at fetch #{ap_id}, #{inspect(e)}", this is now captured and logs as a debug message.

I wanted to add a test too, but I don't know how to test what Logger is used and didn't see examples in test/web/activity_pub/activity_pub_test.exs (I also didn't see a test at all for this function btw)

Closes #783 (closed)