improving error report after PR #115. this adds support for queued and stacked errors.

This is a follow up of PR #115 to control what is happening within the queues that are stacked when shifting multiple modules at once. With this PR, the error report will look like:

```
shifter [info] racing to find the closest .shifter.json file
shifter [info] revving up
shifter [info] racing the directories
shifter [info] using --no-lint
shifter [info] found 102 modules to race, let's do this
shifter [warn] this will be quiet, only status will be emitted for speed. failed builds will print after
.....shifter [err] task: "check" failed; writing zero length file from /Users/caridy/repo/yui3/src/widget-stack/assets/skins/night/widget-stack-skin.css
shifter [err] task: "check" failed; writing zero length file from /Users/caridy/repo/yui3/src/widget-stack/assets/skins/sam/widget-stack-skin.css
shifter [err] build failed, hitting the brakes, fix it and try again!
!.........shifter [err] task: "check" failed; writing zero length file from /Users/caridy/repo/yui3/src/widget/assets/skins/night/widget-base-skin.css
shifter [err] task: "check" failed; writing zero length file from /Users/caridy/repo/yui3/src/widget/assets/skins/sam/widget-base-skin.css
shifter [err] build failed, hitting the brakes, fix it and try again!
!...............shifter [err] task: "check" failed; writing zero length file from /Users/caridy/repo/yui3/src/scrollview/assets/scrollview-base/skins/night/scrollview-base-skin.css
shifter [err] build failed, hitting the brakes, fix it and try again!
!......................................................................
shifter [info] done racing, the gears are toast
shifter [info] finished in 1 minute, 22 seconds, pretty fast huh?
shifter [warn] the following builds exited with a 1
    widget-stack
    widget
    scrollview
shifter [err] Walk failed, 3 builds exited with a 1
```
oops, missing semicolon