Always trap proxy exceptions

CHANGES

@@ -43,6 +43,8 @@ Release date to be decided.
   context which makes them available in imported templates.  One has to be
   very careful with those though because usage outside of macros might
   cause caching.
+- Flask will no longer invoke the wrong error handlers if a proxy
+  exception is passed through.
 
 Version 0.9
 -----------

flask/app.py

@@ -1327,8 +1327,12 @@ class Flask(_PackageBoundObject):
         # ensure not to trash sys.exc_info() at that point in case someone
         # wants the traceback preserved in handle_http_exception.  Of course
         # we cannot prevent users from trashing it themselves in a custom
-        # trap_http_exception method so that's their fault then.
+        # trap_http_exception method so that's their fault then.  Proxy
-        if isinstance(e, HTTPException) and not self.trap_http_exception(e):
+        # exceptions generally must always be trapped (filtered out by
+        # e.code == None)
+        if isinstance(e, HTTPException) \
+           and e.code is not None \
+           and not self.trap_http_exception(e):
             return self.handle_http_exception(e)
 
         blueprint_handlers = ()