Cryptic error when there is nothing to be built with --cache

[F21]

For files that do not need to be built when using --cache, shifter outputs a bunch of weird errors.

I am using shifter 0.4.6 and node 0.10.28.

user@ubuntu:~/Desktop/test/anim2$ shifter --cache --no-lint --no-coverage
shifter [info] racing to find the closest .shifter.json file
shifter [info] revving up
shifter [info] looking for build.json file
shifter [info] found build.json file, shifting
shifter [info] putting the hammer down, let's build this thing!
shifter [warn] down shifting, can't find a meta directory
shifter [info] putting the hammer down
shifter [info] shifting into gear for anim-xyzz
shifter [info] shifting into gear for anim-shape
shifter [info] shifting into gear for anim-scroll
shifter [info] shifting into gear for anim-node-plugin
shifter [info] shifting into gear for anim-easing
shifter [info] shifting into gear for anim-curve
shifter [info] shifting into gear for anim-color
shifter [info] shifting into gear for anim-base
shifter [err] anim-xyzz: [object Object]
shifter [warn] skipping coverage file build due to previous build error
shifter [err] anim-shape: [object Object]
shifter [warn] skipping coverage file build due to previous build error
shifter [err] anim-scroll: [object Object]
shifter [warn] skipping coverage file build due to previous build error
shifter [err] anim-node-plugin: [object Object]
shifter [warn] skipping coverage file build due to previous build error
shifter [err] anim-easing: [object Object]
shifter [warn] skipping coverage file build due to previous build error
shifter [err] anim-curve: [object Object]
shifter [warn] skipping coverage file build due to previous build error
shifter [err] anim-color: [object Object]
shifter [warn] skipping coverage file build due to previous build error
shifter [err] anim-base: [object Object]
shifter [warn] skipping coverage file build due to previous build error
shifter [info] done racing, the gears are toast
shifter [info] finished in 0.019 seconds, pretty fast huh?

I think the error is pretty confusing and misleading. Instead, it would be better if a notice saying the build is already up to date is emitted.

[andrewnicols]

This is happening because of the error callbacks chaining. FTR, here is the actual build fail:

  [ ,
    { task: 'md5check',
      error: 'file has not changed, bailing the build' } ],

This relates to the discussion I started in #122 about callback arguments.