Adding verify phase so that Gatling failure can be failed in verify phase

I am using docker to start an image in pre-integration phase and Gatling runs in integration phase. If Gatling fails I can not clean up docker, I can stop Gatling failing the build but I want to see the build failures. This is a very quick fix, every time Gatling fails write an empty file to results folder and in verify phase check for the existence of the file. If the file is there fail the build. Nothing fancy here just a quick fix to make my usage of this plugin simpler.