Aborting task due to warnings

Warning: Ignoring local @import of ".tmp/public/concat/bootstrap.css" as resource is missing. Use --force to continue.

When I upgraded from 1.0.1 to 2.2.0 The task is aborting because of a warning. 

In the previous version ```Ignoring local @import of "bootstrap.css" as after other CSS content.``` used to happen and the task was completing.