[1.6.x] Fixed #15625 -- Made message in MultiValueDictKeyError less verbose.

Thanks margieroginski for the suggestion.

Backport of 893198509e from master

django/utils/datastructures.py

@@ -298,7 +298,7 @@ class MultiValueDict(dict):
         try:
             list_ = super(MultiValueDict, self).__getitem__(key)
         except KeyError:
-            raise MultiValueDictKeyError("Key %r not found in %r" % (key, self))
+            raise MultiValueDictKeyError(repr(key))
         try:
             return list_[-1]
         except IndexError: