Nicer user errors (first step)

app.py

@@ -17,6 +17,7 @@
 from fastapi.staticfiles import StaticFiles
 
 from common import config
+from common.error import HSError
 from common.session import get_model
 from common.session import get_mongo
 from common.session import get_redis
@@ -130,6 +131,16 @@ async def get_user(
     return await call_next(request)
 
 
+@app.middleware("http")
+async def catch_known_errors(
+    request: Request, call_next: Callable[[Request], Awaitable[Response]]
+) -> Response:
+    try:
+        return await call_next(request)
+    except HSError as e:
+        return JSONResponse(content=str(e), status_code=status.HTTP_400_BAD_REQUEST)
+
+
 @app.get("/health")
 async def health() -> dict[str, str]:
     return {"message": "Healthy!"}