notification email on student unlinking

sails/src/api/models/Supervisor.js

@@ -244,7 +244,7 @@ module.exports = {
     })
     .spread(function (sup, stuSups) {
       if (!stuSups || stuSups.length == 0)
-        throw new Error("No students");
+        return callback(null, []);
       async.eachSeries(stuSups, function(stuSup, next_cb) {
         // set current method and instruction if any
         Student.findOne(stuSup.student.id)