Redirecting error messages from backend

[giulianorasper]

There are cases in which error messages are not redirected to the frontend despite they should be.
E.g. adding an attendee with an already existing email displays the Attendee can not be edited for unknown reasons message whereas a more accurate description is possible since the console displays

An exception occurred while adding a new attendee.
[SQLITE_CONSTRAINT]  Abort due to constraint violation (UNIQUE constraint failed: users.email)

Admins should get as detailed feedback for errors as possible.