[1.7.x] Fixed #24008 -- ValidationError crashes with list of dicts

ValidationError crashes if initialised with list which contains dicts.
Example:

```
>>> ValidationError([{'1': 'one', '2': 'two'}])
Traceback (most recent call last):
  File "<console>", line 1, in <module>
  File "/home/am/.virtualenvs/test/local/lib/python2.7/site-packages/django/core/exceptions.py", line 121, in __init__
    self.error_list.extend(message.error_list)
AttributeError: 'ValidationError' object has no attribute 'error_list'
```