mysql: check for error when getting subtrees

treeTX.getSubtrees iterates over a set of rows from `database/sql`, but
it was failing to check the error status after exiting the for loop that
iterates over them.

On our log deployment, this is causing mistaken error messages of
`preload did not get all tiles` when the real error should be propagated
from mysql (in our case, max_statement_time exceeded).

storage/mysql/tree_storage.go

@@ -244,6 +244,10 @@ func (t *treeTX) getSubtrees(ctx context.Context, treeRevision int64, ids [][]by
 		}
 	}
 
+	if err := rows.Err(); err != nil {
+		return nil, err
+	}
+
 	// The InternalNodes cache is possibly nil here, but the SubtreeCache (which called
 	// this method) will re-populate it.
 	return ret, nil