s3: do not mask failures to retrieve metadata

If a file's metadata is being requested and it cannot be found, this
is a real error. We should allow this exception to bubble up, rather
than masking it.
Some backends (e.g. minio) may provide a `KeyCount` value. If this
is present and zero, assume no records are present, and do not
look for the `Contents` list.

rohmu/object_storage/s3.py

@@ -372,25 +372,23 @@ def iter_key(
             if status_code:
                 self.stats.increase(metric="rohmu.s3.iter_key_response", tags={"status_code": str(status_code)})
 
-            for item in response["Contents"]:
-                if with_metadata:
-                    try:
+            if response.get("KeyCount") != 0:
+                for item in response["Contents"]:
+                    if with_metadata:
                         metadata = {k.lower(): v for k, v in self._metadata_for_key(item["Key"]).items()}
-                    except FileNotFoundFromStorageError:
-                        continue
-                else:
-                    metadata = None
-                name = self.format_key_from_backend(item["Key"])
-                yield IterKeyItem(
-                    type=KEY_TYPE_OBJECT,
-                    value={
-                        "last_modified": item["LastModified"],
-                        "md5": item["ETag"].strip('"'),
-                        "metadata": metadata,
-                        "name": name,
-                        "size": item["Size"],
-                    },
-                )
+                    else:
+                        metadata = None
+                    name = self.format_key_from_backend(item["Key"])
+                    yield IterKeyItem(
+                        type=KEY_TYPE_OBJECT,
+                        value={
+                            "last_modified": item["LastModified"],
+                            "md5": item["ETag"].strip('"'),
+                            "metadata": metadata,
+                            "name": name,
+                            "size": item["Size"],
+                        },
+                    )
 
             for common_prefix in response.get("CommonPrefixes", []):
                 yield IterKeyItem(