couchdb::add_user: get real exit code

using curl, you always get exitcode 0

curl -u "$CREDS" -X PUT 127.0.0.1:5984/leap_web --data ''
{"error":"file_exists","reason":"The database could not be created, the file already exists."}

root@debian-wheezy:~# echo $?
0

(from redmine: created on 2012-11-04, closed on 2013-01-20)