Undeliverable error handling logic for Completable operators

[BharathMG]

Completable.fromAction and Completable.fromRunnable operators were missing RxJavaPlugins calls to handle Undeliverable errors. This behaviour is fixed in this PR.

Also added missing error handling java doc for Completable.fromRunnable.

[codecov]

Codecov Report

Merging #6442 into 2.x will increase coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6442      +/-   ##
============================================
+ Coverage     98.27%   98.29%   +0.02%     
+ Complexity     6297     6295       -2     
============================================
  Files           675      675              
  Lines         45171    45173       +2     
  Branches       6246     6246              
============================================
+ Hits          44391    44402      +11     
  Misses          238      238              
+ Partials        542      533       -9

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Completable.java	100% <ø> (ø)	120 <0> (ø)	⬇️
...l/operators/completable/CompletableFromAction.java	100% <100%> (ø)	4 <0> (ø)	⬇️
...operators/completable/CompletableFromRunnable.java	100% <100%> (ø)	4 <0> (ø)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...nternal/operators/parallel/ParallelReduceFull.java	91.08% <0%> (-3.97%)	2% <0%> (ø)
...tivex/internal/observers/FutureSingleObserver.java	94.33% <0%> (-3.78%)	24% <0%> (-1%)
...tivex/internal/operators/single/SingleTimeout.java	98.33% <0%> (-1.67%)	2% <0%> (ø)
...activex/internal/schedulers/ExecutorScheduler.java	95.97% <0%> (-1.01%)	10% <0%> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.06% <0%> (-0.94%)	2% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	96.58% <0%> (-0.86%)	2% <0%> (ø)
... and 20 more

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