Fix an issue where failure test didn't exists process with code 1

[jihchi]

Environment & versions:

• Mac OS X El Capitan
• nwb 0.6.2
• node v4.2.2

Screenshot:

Travis will always got pass...

[codecov-io]

Current coverage is 93.20%

Merging #36 into master will not affect coverage as of 8d70c8a

@@            master    #36   diff @@
=====================================
  Files           24     24       
  Stmts          427    427       
  Branches         0      0       
  Methods          0      0       
=====================================
  Hit            398    398       
  Partial          0      0       
  Missed          29     29       

Review entire Coverage Diff as of 8d70c8a

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[insin]

D'oh, dumb callback misuse on my part - will merge and release a fixed version this evening

Thanks for the PR, and especially for including a new test case