add license to errors file

Author: andrewseguin

src/cdk/table/table-errors.ts

@@ -1,3 +1,11 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
 /**
  * Returns an error to be thrown when attempting to find an unexisting column.
  * @param id Id whose lookup failed.