Handle promise errors using second argument of then()

[nhunzaker]

What

This commit changes the way Subjects and Microcosm action resolution handle promises such that the error state is passed into the second argument of promise.then().

This prevents a case where callbacks following the success case could raise an exception, but it would be trapped by the .catch() API call. So errors would "disappear" in the following case:

const repo = new Microcosm()

repo.addDomain('test', {
  register() {
    return {
      action: () => {
        throw 'Oops'
      }
    }
  }
})

await repo.push('action') // => where did the error go?

[codecov-io]

Codecov Report

Merging #524 into master will increase coverage by 0.25%. The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master     #524      +/-   ##
==========================================
+ Coverage   93.88%   94.13%   +0.25%     
==========================================
  Files          32       32              
  Lines         801      802       +1     
  Branches      162      162              
==========================================
+ Hits          752      755       +3     
+ Misses         42       40       -2     
  Partials        7        7

Impacted Files	Coverage Δ
packages/microcosm/src/scheduler.js	95.34% <0%> (+4.43%)	:arrow_up:
packages/microcosm/src/coroutine.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/subject.js	100% <100%> (ø)	:arrow_up:
packages/microcosm/src/observable.js	89.16% <93.33%> (+0.18%)	:arrow_up:

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[nhunzaker]

Thanks, @dce !