ValidationError crashes if initialised with a list that includes a ValidationError instance initialised with a dict.

[Sean]

According to the documentation, ValidationError can be initialised with a list of ValidationError instances. However, if the list includes a ValidationError that was initialised with a dictionary then it crashes with the following error: 'ValidationError' object has no attribute 'error_list'.

This bug would occur if someone wanted to raise a ValidationError with a set of field errors together with one or more non-field errors for example.

For example:

from django.core.exceptions import ValidationError
ValidationError([ValidationError({'age', 'age cannot be negative'}), ValidationError('Some other error')])

The problem is caused because when a ValidationError instance is initialised with a dictionary, its error_list attribute is not set (instead error_dict is set) but a ValidationError that is initialised with a list tries to compile a flattened list of all errors using this attribute from each list element. Either ValidationError should be modified to accept a mixture of field and non-field errors (appears to me to be the best solution) or the documentation and error message should be changed to make usage clear.

Related to https://code.djangoproject.com/ticket/23976