Fix for issue 1048: Fixed error message for unrelasing incomplete assignments

``` ruby
raise I18n.t('marking_state.not_complete', :group_name => grouping.group.group_name) if
          submission.get_latest_result.marking_state != Result::MARKING_STATES[:complete] && release
raise I18n.t('marking_state.not_complete_unrelease', :group_name => grouping.group.group_name) if
           submission.get_latest_result.marking_state != Result::MARKING_STATES[:complete] 
```
New condition added to differentiate behavior for "unrelease marks" from "release marks" when the selected rows are incomplete. The error message when unreleasing incomplete assignments now correctly say "can not unrelease result for ...."