Fix internal server error when updating user preferences

[carlobortolan]

When trying to update the user's preferences in the web app, a internal server error is thrown

[carlobortolan]

@github4touchdouble Internal server errors should be handled, even if the feature is not used atm.

So it would be good to leave this open (unless we decide to remove the preferences models & endpoints entirely - then we can close this as not planned)