Change error-handling to be more specific with what it deals with.

I made this change because I'm getting an unreproducable bug in production
which causes duplicate NetInfo, Source, User, Machine, etc objects to be
created.

The old error-handling code just caught all Exceptions. Thus, the duplicate
NetInfo/User/Machine/Action/etc problem we're having could be the result of
some exception other than ObjectDoesNotExist.