Grade retrieval fails for students who are not enrolled

[alexhorn]

Students who are not enrolled (because they haven't started officially or because they have finished their studies) get "Error: Service not available (-112)" when opening /personal. This is due to the grades method not being available. I kind of fixed it in 4d9f9c9, but it needs to be reworked.

[M4GNV5]

To me it seems like the grades endpoint is generally unavailable at the moment(?)

Anyways: It's not just the grades, for graduated accounts the majority of "personal data" seems empty too:

We should probably go with a employee-like display of /personal?