Handle missing data more gracefully

src/butlerexampleservice/handlers/external.py

@@ -15,6 +15,7 @@
 external_router = APIRouter()
 """FastAPI router for all external handlers."""
 
+from fastapi import HTTPException
 from lsst.daf.butler import LabeledButlerFactory
 from safir.dependencies.gafaelfawr import auth_delegated_token_dependency
 
@@ -33,6 +34,8 @@ def get_coadd_url(
     )
 
     ref = butler.find_dataset("deepCoadd", dataId={"tract": tract, "patch": patch})
+    if ref is None:
+        raise HTTPException(status_code=404, detail="Coadd not found")
     return str(butler.getURI(ref))