need to fail the task if copying fails, it was just silently exiting

Copying fails with current version of bower, see https://github.com/bower/bower/pull/784

Bower list --paths reported the main files as a comma separated string, not an array. And grunt-bower-task tries to copy an unsplit string. The issue is fixed in bower, but anyway grunt-bower-task should fail. See this travis CI output for how it currently behaves. It does not fail when copying fails. https://travis-ci.org/mokkabonna/knockout.bindingHandlers.trimmedValue/builds/10326378