Better exception handling


It seems like there are some unhandled exceptions, that are being handled downstream (viewser) rather than when they are raised. Apart from cluttering the log stream, this can also lead to unpredictable behaviour.

I noticed this when I overwrote a queryset (SQL error, non-unique PK). I should investigate here, and in ViEWSER how this situation is handled in the code, and handle it more elegantly.
