[1.9.x] Followed recommended ValidationError use in docs.

Backport of 0db7e61076116c2d93d61f98ef31690542359e48 from master

docs/howto/custom-model-fields.txt

@@ -464,6 +464,7 @@ instances::
 
     from django.core.exceptions import ValidationError
     from django.db import models
+    from django.utils.translation import ugettext_lazy as _
 
     def parse_hand(hand_string):
         """Takes a string of cards and splits into a full hand."""
@@ -471,7 +472,7 @@ instances::
         p2 = re.compile('..')
         args = [p2.findall(x) for x in p1.findall(hand_string)]
         if len(args) != 4:
-            raise ValidationError("Invalid input for a Hand instance")
+            raise ValidationError(_("Invalid input for a Hand instance"))
         return Hand(*args)
 
     class HandField(models.Field):

docs/ref/validators.txt

@@ -16,10 +16,14 @@ different types of fields.
 For example, here's a validator that only allows even numbers::
 
     from django.core.exceptions import ValidationError
+    from django.utils.translation import ugettext_lazy as _
 
     def validate_even(value):
         if value % 2 != 0:
-            raise ValidationError('%s is not an even number' % value)
+            raise ValidationError(
+                _('%(value)s is not an even number'),
+                params={'value': value},
+            )
 
 You can add this to a model field via the field's :attr:`~django.db.models.Field.validators`
 argument::