Error messaging fix

Bart Hofman
2019-04-10T22:11:45+02:00

Wrong error code when permission denied

### Current behaviour / Reproducing the bug

1. user with only event change permissions
2. Try to change a registration of an event
3. errors "Correct the error below" or "Corrigeer de fouten hieronder"

### Expected behaviour

1. user with only event change permissions
2. Try to change a registration of an event
3. errors specific error message that fits the error