Fixed #20463 -- Made get_or_create more robust.

When an exception other than IntegrityError was raised, get_or_create
could fail and leave the database connection in an unusable state.

Thanks UloPe for the report.

django/db/models/query.py

@@ -9,7 +9,7 @@ import warnings
 
 from django.conf import settings
 from django.core import exceptions
-from django.db import connections, router, transaction, IntegrityError
+from django.db import connections, router, transaction, DatabaseError
 from django.db.models.constants import LOOKUP_SEP
 from django.db.models.fields import AutoField
 from django.db.models.query_utils import (Q, select_related_descend,
@@ -382,13 +382,13 @@ class QuerySet(object):
                 obj.save(force_insert=True, using=self.db)
                 transaction.savepoint_commit(sid, using=self.db)
                 return obj, True
-            except IntegrityError:
+            except DatabaseError:
                 transaction.savepoint_rollback(sid, using=self.db)
                 exc_info = sys.exc_info()
                 try:
                     return self.get(**lookup), False
                 except self.model.DoesNotExist:
-                    # Re-raise the IntegrityError with its original traceback.
+                    # Re-raise the DatabaseError with its original traceback.
                     six.reraise(*exc_info)
 
     def _earliest_or_latest(self, field_name=None, direction="-"):