Check for endpoints in response, displaying error message properly
Root containers are droppable.
Merge pull request #556 from jaycee/drag-and-drop-mv-reloads-page

Root containers are droppable.
Merge pull request #555 from makyo/add-subrel

Display an error if Juju will not allow a subordinate relation.

Check for endpoints in response, displaying error message properly; fixes 1369606