Fixed render error of required nested serializer.

Fixes an error that occurs on rendering response with error that says that required nested serializer field must be specified in data.

rest_framework/utils/serializer_helpers.py

@@ -101,7 +101,7 @@ class NestedBoundField(BoundField):
     def __getitem__(self, key):
         field = self.fields[key]
         value = self.value.get(key) if self.value else None
-        error = self.errors.get(key) if self.errors else None
+        error = self.errors.get(key) if isinstance(self.errors, dict) else None
         if hasattr(field, 'fields'):
             return NestedBoundField(field, value, error, prefix=self.name + '.')
         return BoundField(field, value, error, prefix=self.name + '.')