NotAuthorisedError may unwrap to os.ErrNotExist.

Recent versions of Go do it at startup, so only print
a warning if the limit is too low.

Also fix return code in WHIP.

The group and name are in the path, this avoids redundancies.

It was only being enforced in the client.

Rename the fallback-users entry to wildcard-user, and only
allow a single fallback user.  This is missing the HTTP API.

Suggested by Dianne Skoll.