Fixed #21619 -- Made SingleObjectMixin.get_object catch a more precise exception.

Thanks to Keryn Knight for the report.

django/views/generic/detail.py

@@ -1,6 +1,6 @@
 from __future__ import unicode_literals
 
-from django.core.exceptions import ImproperlyConfigured, ObjectDoesNotExist
+from django.core.exceptions import ImproperlyConfigured
 from django.db import models
 from django.http import Http404
 from django.utils.translation import ugettext as _
@@ -50,7 +50,7 @@ class SingleObjectMixin(ContextMixin):
         try:
             # Get the single item from the filtered queryset
             obj = queryset.get()
-        except ObjectDoesNotExist:
+        except queryset.model.DoesNotExist:
             raise Http404(_("No %(verbose_name)s found matching the query") %
                           {'verbose_name': queryset.model._meta.verbose_name})
         return obj