ErrMachineNotExist msg: Fix grammar, more helpful
ErrMachineNotExist msg: Fix grammar, more helpful

This adds the word "does" to make it sound nicer and it also asks if the
user ran `boot2docker init`.

SvenDowideit/boot2docker-cli,boot2docker/boot2docker-cli,SvenDowideit/boot2docker-cli,moxiegirl/boot2docker-cli,mattrobenolt/boot2docker-cli,beni55/boot2docker-cli,boot2docker/boot2docker-cli,beni55/boot2docker-cli,mattrobenolt/boot2docker-cli,moxiegirl/boot2docker-cli