Checklist

• [ ✓] I have verified that that issue exists against the master branch of Django REST framework.
• [ ✓] I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• [✓ ] This is not a usage question. (Those should be directed to the discussion group instead.)
• [✓ ] This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• [ ✓] I have reduced the issue to the simplest possible case.
• [ ] I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

from rest_framework import serializers

class CustomSerializer(serializers.Serializer):
    f1 = serializers.IntegerField()
    f2 = serializers.CharField()

    def to_internal_value(self, data):
        raise serializers.ValidationError('Never Valid!')


s = CustomSerializer(data={'f1': 1, 'f2': 2})
if not s.is_valid():
    print(s.errors)

Expected behavior

Get the validation errors dict

Actual behavior

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
~/bug_report.py in <module>()
     11 s = CustomSerializer(data={'f1': 1, 'f2': 2})
     12 if not s.is_valid():
---> 13     print(s.errors)

/usr/local/lib/python3.6/site-packages/rest_framework/serializers.py in errors(self)
    543             detail = ErrorDetail('No data provided', code='null')
    544             ret = {api_settings.NON_FIELD_ERRORS_KEY: [detail]}
--> 545         return ReturnDict(ret, serializer=self)
    546 
    547 

/usr/local/lib/python3.6/site-packages/rest_framework/utils/serializer_helpers.py in __init__(self, *args, **kwargs)
     19     def __init__(self, *args, **kwargs):
     20         self.serializer = kwargs.pop('serializer')
---> 21         super(ReturnDict, self).__init__(*args, **kwargs)
     22 
     23     def copy(self):

ValueError: too many values to unpack (expected 2)

Technically, the docs do say (http://www.django-rest-framework.org/api-guide/serializers/#advanced-serializer-usage)

If any of the validation fails, then the method should raise a serializers.ValidationError(errors).
Typically the errors argument here will be a dictionary mapping field names to error messages.

but at the same time in another place a string is passed into the exception in the docs (http://www.django-rest-framework.org/api-guide/exceptions/#validationerror)

The detail argument may be a list or dictionary of error details, and may also be a nested data structure. By convention you should import the serializers module and use a fully qualified ValidationError style, in order to differentiate it from Django's built-in validation error. For example. raise serializers.ValidationError('This field must be an integer value.')

The exception above happens when serializer is trying to build an OrderedDict from a list of one error. Either ValidationError needs to do a better type coercion (it converts details argument to a list, but it expects a dict or a list with keys and values) or it should not allow string literal as details.