Update error responses according to Fleetlock spec

[olivierlemasle]

• Update error responses according to Fleetlock spec (JSON output)

  Cf. https://coreos.github.io/zincati/development/fleetlock/protocol/#errors

• Update Fleetlock specification location

Remarks:

• According to Fleetlock spec, the error kinds must have a bounded/small cardinality. If you have suggestions regarding the kinds or their wording, please let me know.
• I've added the testify library. I do not like bringing new test dependencies, however assert.JSONEq is quite useful. I can drop the library if you prefer.
• I've fixed a small error: in error message

  fleetlock/internal/server.go

  Line 155 in c5038dc

  s.log.WithFields(fields).Errorf("fleetlock: error obtaining reboot lease: %v", err)

  
  err was nil because it was defined in the if statement.

[dghubble]

Thanks for looking at this. Left some comments. Also, can you structure your commits to have the fleetlock error encoding change in one commit, and the vendor in the second commit?

[dghubble]

Thanks