[1.8.x] Caught all exceptions raised by Engine.get_default().

In addition to ImproperlyConfigured, Engine.get_default() may also raise
ImportError or other exceptions. It's better to catch all exceptions in
places where the default engine isn't strictly required.

Backport of 27f9ff45 from master

django/contrib/admin/sites.py

@@ -177,8 +177,14 @@ class AdminSite(object):
                 "setting in order to use the admin application.")
         try:
             default_template_engine = Engine.get_default()
-        except ImproperlyConfigured:
-            # Skip the check if the user has a non-trivial TEMPLATES setting
+        except Exception:
+            # Skip this non-critical check:
+            # 1. if the user has a non-trivial TEMPLATES setting and Django
+            #    can't find a default template engine
+            # 2. if anything goes wrong while loading template engines, in
+            #    order to avoid raising an exception from a confusing location
+            # Catching ImproperlyConfigured suffices for 1. but 2. requires
+            # catching all exceptions.
             pass
         else:
             if ('django.contrib.auth.context_processors.auth'

django/views/debug.py

@@ -7,7 +7,6 @@ import sys
 import types
 
 from django.conf import settings
-from django.core.exceptions import ImproperlyConfigured
 from django.core.urlresolvers import resolve, Resolver404
 from django.http import (HttpResponse, HttpResponseNotFound, HttpRequest,
     build_request_repr)
@@ -282,7 +281,11 @@ class ExceptionReporter(object):
         """Return a dictionary containing traceback information."""
         try:
             default_template_engine = Engine.get_default()
-        except ImproperlyConfigured:
+        except Exception:
+            # Since the debug view must never crash, catch all exceptions.
+            # If Django can't find a default template engine, get_default()
+            # raises ImproperlyConfigured. If some template engines fail to
+            # load, any exception may be raised.
             default_template_engine = None
 
         # TODO: add support for multiple template engines (#24120).