Add a generic error page for all HTTP errors

Previously the site would give an unstyled web-server error on a lot of
errors (such as trying to visit the page for a username that doesn't
exist). This just adds a generic error page, but we'll want to add some
more specialized ones for certain errors.

tildes/tildes/templates/error_403.jinja2 → tildes/tildes/templates/error_page.jinja2

@@ -3,12 +3,13 @@
 
 {% extends 'base_no_sidebar.jinja2' %}
 
-{% block title %}Error 403 (Forbidden){% endblock %}
+{% block title %}{{ error }}{% endblock %}
 
 {% block content %}
 <div class="empty">
-  <h2 class="empty-title">Error 403 (Forbidden)</h2>
-  <p class="empty-subtitle">You don't have access to this page.</p>
+  <h2 class="empty-title">{{ error }}</h2>
+  <p class="empty-subtitle">{{ description }}</p>
+
   <div class="empty-action"><a href="/" class="btn btn-primary">Back to the home page</a></div>
 </div>
 {% endblock %}

tildes/tildes/views/exceptions.py

@@ -3,21 +3,18 @@
 
 """Views used by Pyramid when an exception is raised."""
 
-from pyramid.httpexceptions import HTTPNotFound
+from pyramid.httpexceptions import HTTPException, HTTPForbidden, HTTPNotFound
 from pyramid.request import Request
-from pyramid.view import forbidden_view_config, exception_view_config
+from pyramid.view import (
+    exception_view_config,
+    forbidden_view_config,
+    notfound_view_config,
+)
 from sqlalchemy import cast, desc, func, Text
 
 from tildes.models.group import Group
 
 
-@forbidden_view_config(xhr=False, renderer="error_403.jinja2")
-def forbidden(request: Request) -> dict:
-    """403 Forbidden page."""
-    request.response.status_int = 403
-    return {}
-
-
 @exception_view_config(
     HTTPNotFound, route_name="group", renderer="error_group_not_found.jinja2"
 )
@@ -34,3 +31,20 @@ def group_not_found(request: Request) -> dict:
         .all()
     )
     return {"group_suggestions": group_suggestions, "supplied_name": supplied_name}
+
+
+@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")
+def generic_error_page(request: Request) -> dict:
+    """Display a generic error page for all HTTP exceptions."""
+    request.response.status_int = request.exception.status_int
+
+    error = f"Error {request.exception.status_code} ({request.exception.title})"
+
+    if isinstance(request.exception, HTTPForbidden):
+        description = "You don't have access to this page"
+    else:
+        description = request.exception.explanation
+
+    return {"error": error, "description": description}