Fixed #10863 -- Added HTML support to mail_managers() and mail_admins(), and used this to provide more and prettier detail in error emails. Thanks to boxed for the suggestion, and to Rob Hudson and Brodie Rao for their work on the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14844 bcc190cf-cafb-0310-a4f2-bffc1f526a37