Adding a basic auth user that already exists results in 500 errro

[brentgriffin]

Running Janus in K8s cluster with cassandra storage. When adding a basic auth user that already exists, receive a HTTP 500 response and the body reads:

"runtime error: invalid memory address or nil pointer dereference"

Updating the user password would be good but if we want to error here, a more meaningful response body would be helpful.