Retry fetching users with fewer users on errors

Apparently it varies how many users can be fetched in one request with
the users.info method. I have had successful requests with 4096 users,
while some people have reported getting HTTP 500 when fetching 1000
users and one person got an error response with error `too_many_users`
when fetching 500 users.

So when we get either of those errors, reduce the number of users we
fetch in one request and try again until it works or the number of users
has been reduced to 1.

Fixes #924

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