3.x: Fix many operators swallowing undeliverable exceptions

[akarnokd]

Fix many operators to emit the collected exception in case of a cancel/dispose call to the plugin error handler (i.e., becoming undeliverable excptions).

In addition, the terminal event/error delivery has been unified in many other operators so that the terminal-indicator exception is never leaked.

There will be a separate PR about cleaning up the addThrowable usage cases (#6611).

Fixes: #6587

[codecov]

Codecov Report

Merging #6612 into 3.x will decrease coverage by 0.01%.
The diff coverage is 97.9%.

@@             Coverage Diff              @@
##                3.x    #6612      +/-   ##
============================================
- Coverage     98.17%   98.15%   -0.02%     
- Complexity     6180     6186       +6     
============================================
  Files           678      678              
  Lines         44910    44771     -139     
  Branches       6214     6172      -42     
============================================
- Hits          44091    43947     -144     
- Misses          275      279       +4     
- Partials        544      545       +1

Impacted Files	Coverage Δ	Complexity Δ
...nal/operators/flowable/FlowableMergeWithMaybe.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...nal/operators/mixed/ObservableConcatMapSingle.java	100% <100%> (ø)	3 <0> (ø)	⬇️
.../operators/mixed/FlowableSwitchMapCompletable.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...al/operators/flowable/FlowableMergeWithSingle.java	100% <100%> (ø)	2 <0> (ø)	⬇️
...ava/io/reactivex/observers/SerializedObserver.java	100% <100%> (ø)	27 <1> (ø)	⬇️
...ators/observable/ObservableFlatMapCompletable.java	95.71% <100%> (-0.24%)	2 <0> (ø)
...perators/observable/ObservableMergeWithSingle.java	99.07% <100%> (-0.93%)	2 <0> (ø)
...va/io/reactivex/internal/util/AtomicThrowable.java	100% <100%> (ø)	23 <15> (+15)	⬆️
...lowable/FlowableFlatMapCompletableCompletable.java	96.15% <100%> (-0.24%)	3 <0> (ø)
...ternal/operators/mixed/FlowableSwitchMapMaybe.java	100% <100%> (ø)	2 <0> (ø)	⬇️
... and 59 more

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[artem-zinnatullin]

Nice