Error page: cover HTTPError, not HTTPException

HTTPException hits all HTTP codes, including redirections, which was
breaking the tild.es shortener. Using HTTPError instead will only cover
the 4xx and 5xx codes, which is what we want.

tildes/tildes/views/exceptions.py

@@ -3,7 +3,7 @@
 
 """Views used by Pyramid when an exception is raised."""
 
-from pyramid.httpexceptions import HTTPException, HTTPForbidden, HTTPNotFound
+from pyramid.httpexceptions import HTTPError, HTTPForbidden, HTTPNotFound
 from pyramid.request import Request
 from pyramid.view import (
     exception_view_config,
@@ -35,7 +35,7 @@ def group_not_found(request: Request) -> dict:
 
 @notfound_view_config(xhr=False, renderer="error_page.jinja2")
 @forbidden_view_config(xhr=False, renderer="error_page.jinja2")
-@exception_view_config(context=HTTPException, xhr=False, renderer="error_page.jinja2")
+@exception_view_config(context=HTTPError, xhr=False, renderer="error_page.jinja2")
 def generic_error_page(request: Request) -> dict:
     """Display a generic error page for all HTTP exceptions."""
     request.response.status_int = request.exception.status_int