Handle jscs errors

* Add failure handler to the promise for major errors
  during the jscs process (it is a promise, not a plain callback,
  so it can fail).

* Call grunt.log.error instead of grunt.log.ok when reporting
  the number of failures (makes the >> red instead of green
  in the output, similar to how other grunt plugins report).

* Rename variable 'i' to 'completed' to more clearly indicate
  it is incremented when an iteration is finished, not when
  it begins.

* At new line at EOF.

Fixes #15.