Enrollment states the token is invalid

[teon]

1. Added a new user
2. Started manual enrollment
3. Open enrollment WEB (for example our dev instance or production) - not in the desktop client
4. Entered the token provided and got:

[j-chmielewski]

This was caused by busy loop in proxy after core disconnected. The invalid state after core disconnection is fixed in #78.

Additionally errors should be made more precise on the frontend to discriminate between "Invalid token" and any other backend error.