Fix webmodels mandatory type field regression

[mdedetrich]

This PR does 2 things

1. Does some code cleanups (i.e. making some case objects final as well as minor adjustments to logging
2. Fixes a regression introduced by Update webmodels version #132 . That PR updates webmodels which now has type as a mandatory field. Even though the Nakadi API has type as a mandatory field for Problem, this doesn't reflect in all errors that Nakadi returns which means that errors such as NoEmptySlotsOrCursorReset didn't serialize properly because we were expecting a Problem with a type field. A test was also added to verify this. An issue at Make NoEmptySlotsOrCursorReset extend GeneralError again #134 has been made to track this.

[codecov-io]

Codecov Report

Merging #133 into master will not change coverage by %.
The diff coverage is 66.66%.

@@           Coverage Diff           @@
##           master     #133   +/-   ##
=======================================
  Coverage   75.77%   75.77%           
=======================================
  Files          22       22           
  Lines         516      516           
  Branches       13       13           
=======================================
  Hits          391      391           
  Misses        125      125           

Impacted Files	Coverage Δ
...c/main/scala/org/zalando/kanadi/api/Registry.scala	0.00% <ø> (ø)
...n/scala/org/zalando/kanadi/api/Subscriptions.scala	69.73% <0.00%> (ø)
...main/scala/org/zalando/kanadi/api/EventTypes.scala	94.69% <100.00%> (ø)
src/main/scala/org/zalando/kanadi/api/Events.scala	79.20% <100.00%> (ø)

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[itachi3]

👍