Translate db messages to a user-understandable form
Add flash message handling to ModalSelector

* also use stopPropagation to prevent duplicate messages
Slightly improve error messaging for ModalSelector mapping
Merge pull request #359 from reciprocity/feature/better_error_messages

56731462 Translate db messages to a user-understandable form
Fix display of mappings of People/References to Program
Handle unmapping of controls correctly

* Previously, unmapping a Control on a Directive page caused the
  deletion of the Directive.  This is fixed.
* Ensure Controls mapped to Directives show up in Program page's "Mapped
  Controls" widget
* Special case for Control unmapping now doesn't apply only on Directive
  page.