Supress counting not-found as an error when reading alertmanager state. (cortexproject#4156)

This error is expected when a new user is configured and has not
persisted any state yet, or if an alertmanager is being transitioned
from non-sharding to sharding operation.

Signed-off-by: Steve Simpson <steve.simpson@grafana.com>

Author: stevesg

pkg/alertmanager/alertstore/bucketclient/bucket_client.go

@@ -192,5 +192,5 @@ func (s *BucketAlertStore) getUserBucket(userID string) objstore.Bucket {
 }
 
 func (s *BucketAlertStore) getAlertmanagerUserBucket(userID string) objstore.Bucket {
-	return bucket.NewUserBucketClient(userID, s.amBucket, s.cfgProvider)
+	return bucket.NewUserBucketClient(userID, s.amBucket, s.cfgProvider).WithExpectedErrs(s.amBucket.IsObjNotFoundErr)
 }