[1.2.X] Fixed sending mail on 404s to fail silently.

A failure in the e-mail system should not turn a 404 into a 500.

Backport of [15145] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15146 bcc190cf-cafb-0310-a4f2-bffc1f526a37