Describe the issue

If more than one Person matches the email_or_name argument to lookup_community_list() and one or more of those Persons has no user, an unexpected exception occurs. This usually results in a 500 (server error) response.

The method is trying to populate a MultiplePersonError with a list of usernames that can be used to put together a list of disambiguated options for a 300 status code response. That's a helpful spirit, but given that we can't necessarily support that feature in general (and because it's unclear that browsers will actually do helpful things with the 300), I think it'd be better just to return a 404. If someone has trouble accessing their community list, they'll ask for help.

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