Do not present a provider without a login as the selected walkthrough
model, and grey out Generate when readiness is false instead of showing
a login-error blocker or raw auth banner.
Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>
When the walkthrough small model resolves to a provider with no usable
login, readiness was still ready and generate returned a raw 500 message.
Refuse up front with no-provider-login and surface a blocker instead.
Closesopenchamber/openchamber#2607
Co-authored-by: Serhii Dziupin <makeittech@users.noreply.github.com>