store feed errors

storage/storage.go

@@ -65,12 +65,16 @@ end;
 create table if not exists http_states (
  feed_id        references feeds(id) unique,
  last_refreshed datetime not null,
- last_error     string,
 
  -- http header fields --
  last_modified  string not null,
  etag           string not null
 );
+
+create table if not exists feed_errors (
+ feed_id        references feeds(id) unique,
+ error          string
+);
 `
 
 type Storage struct {