Add user/group/class catch against blank and error cleanup

- On the collections and media object page, if the user clicks add for a user, group or class, avalon will flash an error message saying that field shouldn't be blank and will prevent a blank from being saved.

- There was inconsistent naming of flash[:error] and flash[:errors]. Changed them all to flash[:error]
Replace more flash[:errors] with flash[:error] and add back ArguementError catch
Merge pull request #274 from avalonmediasystem/bugfix/VOV-2526

Add user/group/class catch against blank and error cleanup