Fixed #16770 -- Eliminated TemplateSyntaxError wrapping of exceptions. Thanks to Justin Myles-Holmes for report and draft patch.

Exceptions raised in templates were previously wrapped in TemplateSyntaxError
(in TEMPLATE_DEBUG mode only) in order to provide template source details on
the debug 500 page. The same debug information is now provided by annotating
exceptions rather than wrapping them. This makes catching exceptions raised
from templates more sane, as it's consistent in or out of DEBUG, and you can
catch the specific exception(s) you care about rather than having to also catch
TemplateSyntaxError and unwrap it.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16833 bcc190cf-cafb-0310-a4f2-bffc1f526a37