Update the content of self._errors.
The field argument is the name of the field to which the errors should be added. If it’s None, treat the errors as NON_FIELD_ERRORS.
The error argument can be a single error, a list of errors, or a dictionary that maps field names to lists of errors. An “error” can be either a simple string or an instance of ValidationError with its message attribute set and a “list or dictionary” can be an actual list or dict or an instance of ValidationError with its error_list or error_dict attribute set.
If error is a dictionary, the field argument must be None and errors will be added to the fields that correspond to the keys of the dictionary.