Improve auth error handling

[naltatis]

fixes #14373

settings api now return error when not initialized

• auth endpoints return status 500 when database isn't initialized
• better error communication to user
  • "server error" instead of "wrong password"
  • no "create admin password" when db isn't configured

For later: introduce same behavior for SetString

[andig]

no "create admin password" when db isn't configured

how could this happen?

[naltatis]

"Configured" is wrong. I meant initialized. (Startup error)

[andig]

Lass mich bitte nochmal drüber nachdenken, das ist viel hin-und-her