Fixed #17281 -- Prevented `AdminErrorHandler` from silently failing if the log message contains newlines. Thanks to Russell Keith-Magee for the report and to Bartolome Sanchez Salado and Marcin Wróbel for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17501 bcc190cf-cafb-0310-a4f2-bffc1f526a37