Raise informative error if region detection fails (#558)

* Raise informative error if region detection fails

* Fix test

Author: maxrjones

virtualizarr/manifests/store.py

@@ -131,7 +131,12 @@ def _find_bucket_region(bucket_name: str) -> str:
     import requests
 
     resp = requests.head(f"https://{bucket_name}.s3.amazonaws.com")
-    return resp.headers["x-amz-bucket-region"]
+    region = resp.headers.get("x-amz-bucket-region")
+    if not region:
+        raise ValueError(
+            f"Unable to automatically determine region for bucket {bucket_name}"
+        )
+    return region
 
 
 def default_object_store(filepath: str) -> ObjectStore:

virtualizarr/tests/test_manifests/test_store.py

@@ -161,6 +161,15 @@ def test_default_object_store_local(tmpdir):
     assert isinstance(store, LocalStore)
 
 
+@requires_obstore
+def test_default_region_raises():
+    file = "s3://cworthy/oae-efficiency-atlas/data/experiments/000/01/alk-forcing.000-1999-01.pop.h.0347-01.nc"
+    with pytest.raises(
+        ValueError, match="Unable to automatically determine region for bucket*"
+    ):
+        default_object_store(file)
+
+
 @requires_obstore
 class TestManifestStore:
     def test_manifest_store_properties(self, local_store):