ModelReprMixin: Fix handling of missing __unicode__ function.

The old behavior was to raise an exception, but Django was catching
the exception and doing unexpected things. For instance, in the
manage.py shell, printing out a ModelReprMixin object (with
__unicode__ not implemented) would result in nothing being printed,
rather than it raising a error or otherwise alerting the programmer as
to what was going on.

zerver/lib/str_utils.py

@@ -83,7 +83,9 @@ class ModelReprMixin(object):
     """
     def __unicode__(self):
         # type: () -> text_type
-        raise NotImplementedError("__unicode__ is not implemented in subclass of ModelReprMixin")
+        # Originally raised an exception, but Django (e.g. the ./manage.py shell)
+        # was catching the exception and not displaying any sort of error
+        return u"Implement __unicode__ in your subclass of ModelReprMixin!"
 
     def __str__(self):
         # type: () -> str