Include a message in the teams.ImmutableMembershipFieldException

Also add a __repr__ to CourseTeam so that it could be useful too.

lms/djangoapps/teams/models.py

@@ -130,6 +130,9 @@ class CourseTeam(models.Model):
 
         return course_team
 
+    def __repr__(self):
+        return "<CourseTeam team_id={0.team_id}>".format(self)
+
     def add_user(self, user):
         """Adds the given user to the CourseTeam."""
         if not CourseEnrollment.is_enrolled(user, self.course_id):
@@ -172,7 +175,9 @@ class CourseTeamMembership(models.Model):
             # an immutable field.
             current_value = getattr(self, name, None)
             if current_value is not None:
-                raise ImmutableMembershipFieldException
+                raise ImmutableMembershipFieldException(
+                    "Field %r shouldn't change from %r to %r" % (name, current_value, value)
+                )
         super(CourseTeamMembership, self).__setattr__(name, value)
 
     def save(self, *args, **kwargs):