HTML error pages for FastAPI

Signed-off-by: Lukas Fleischer <lfleischer@archlinux.org>

aurweb/asgi.py

-from fastapi import FastAPI
+import http
+
+from fastapi import FastAPI, HTTPException
+from fastapi.responses import HTMLResponse
 from starlette.middleware.sessions import SessionMiddleware
 
 import aurweb.config
@@ -14,3 +17,14 @@ if not session_secret:
 app.add_middleware(SessionMiddleware, secret_key=session_secret)
 
 app.include_router(sso.router)
+
+
+@app.exception_handler(HTTPException)
+async def http_exception_handler(request, exc):
+    """
+    Dirty HTML error page to replace the default JSON error responses.
+    In the future this should use a proper Arch-themed HTML template.
+    """
+    phrase = http.HTTPStatus(exc.status_code).phrase
+    return HTMLResponse(f"<h1>{exc.status_code} {phrase}</h1><p>{exc.detail}</p>",
+                        status_code=exc.status_code)