Yo dawg I heard you like errors... well the ErrorView itself was causing errors making it unable to send a response.
It turns out ErrorView couldn't render properly because of no function clause matching in Plug.Conn.resp/3
That function clause? is_binary(body)
Instead, a Map was being passed into the body. It should have been encoded by Jason first. I looked at the Conn, and it had format: "activity+json". It turns out Phoenix needs to know about this if it wants to render the error. It only knows about regular "json" so far.
The fix is simple: just tell Phoenix about "activity+json" by adding it to the config: https://stackoverflow.com/a/48986889