Protect constant callback

[benblack86]

I was playing around with callback, because I hear they are like the future 🥁 .

The docs say that CallbackExecutionException is thrown if there is an uncaught exception in the execution block of a Callback. This is true for unmapped and mapped callback, but not for constant callback. I added a try/catch!

@DanSimon @nsauro @dxuhuang @aliyakamercan

[codecov-io]

Codecov Report

Merging #581 into develop will increase coverage by 0.02%. The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #581      +/-   ##
===========================================
+ Coverage     84.9%   84.92%   +0.02%     
===========================================
  Files           99       99              
  Lines         4431     4432       +1     
  Branches       367      367              
===========================================
+ Hits          3762     3764       +2     
+ Misses         669      668       -1

Impacted Files	Coverage Δ
...sus/src/main/scala/colossus/service/Callback.scala	88.37% <100%> (+0.09%)	:arrow_up:
...s/src/main/scala/colossus/metrics/Collection.scala	100% <0%> (+2.56%)	:arrow_up:

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[jlbelmonte]

👍