_safe_revert: Add a logging traceback for IntegrityError

This will be helpful @reversion.register users that explicitly declare
their fields.

Fixes #750

reversion/models.py

@@ -1,5 +1,6 @@
 from collections import defaultdict
 from itertools import chain, groupby
+import logging
 
 import django
 from django.apps import apps
@@ -22,13 +23,17 @@
                                  _get_content_type, _get_options)
 
 
+logger = logging.getLogger(__name__)
+
+
 def _safe_revert(versions):
     unreverted_versions = []
     for version in versions:
         try:
             with transaction.atomic(using=version.db):
                 version.revert()
         except (IntegrityError, ObjectDoesNotExist):
+            logger.exception('Could not revert to {version}')
             unreverted_versions.append(version)
     if len(unreverted_versions) == len(versions):
         raise RevertError(gettext("Could not save %(object_repr)s version - missing dependency.") % {