Improve error message when removing non-existent objects

Author: umago

jsonpatch.py

@@ -399,7 +399,8 @@ def apply(self, obj):
         try:
             del subobj[part]
         except IndexError as ex:
-            raise JsonPatchConflict(str(ex))
+            msg = "can't remove non-existent object '{0}'".format(part)
+            raise JsonPatchConflict(msg)
 
         return obj