bAdCAsE error

This splits the old `Incorrect password` error to add a separate error for `Incorrect email case`. When the auth fails and the request email doesn't match the database's case it will return this new error.

The new errno is 120 with status code 400.

r? @rfk @chilts

cc @ncalexan

I'm lazy and did this on top of #521 so lets please review/merge that one first.