failure should indicate why it failed via RESTful API

[chillum]

API should return the reason, why the check failed. This may help when the initial reason is not more the case. Example (not sure about exact syntax yet):

[
  {
    "name": "Test",
    "web": "http://localhost",
    "failed": true,
    "why": "Get 127.0.0.1 failed"
  },
]

We probably should introduce a limit on reason's length (also applies to match).

[malud]

See PR #17 I have added a new field error which contains the related err message.