2.x: explain null observer/subscriber error reports in detail from RxJavaPlugins

[punitda]

This PR is related to issue #6172.
With these changes, all reactive types will emit detailed error message when onSubscribe hook installed using RxJavaPlugins returns null observer/subscriber:

Observable

"The RxJavaPlugins.onSubscribe hook returned a null Observer. Please check the handler provided to RxJavaPlugins.setOnObservableSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins"

Flowable

The RxJavaPlugins.onSubscribe hook returned a null FlowableSubscriber. Please check the handler provided to RxJavaPlugins.setOnFlowableSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins

Single

The RxJavaPlugins.onSubscribe hook returned a null SingleObserver. Please check the handler provided to RxJavaPlugins.setOnSingleSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins

Maybe

The RxJavaPlugins.onSubscribe hook returned a null MaybeObserver. Please check the handler provided to RxJavaPlugins.setOnMaybeSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins

Completable

The RxJavaPlugins.onSubscribe hook returned a null CompletableObserver. Please check the handler provided to RxJavaPlugins.setOnCompletableSubscribe for invalid null returns. Further reading: https://github.com/ReactiveX/RxJava/wiki/Plugins

Note :
There was no null check on observer returned from RxJavaPlugins.onSubscribe(this, observer) method call in Completable's subscribe() method.
So, I've added it in this PR. Is that fine?

[akarnokd]

👍

[codecov]

Codecov Report

Merging #6175 into 2.x will decrease coverage by 0.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6175      +/-   ##
============================================
- Coverage     98.23%   98.22%   -0.02%     
- Complexity     6196     6199       +3     
============================================
  Files           667      667              
  Lines         44879    44880       +1     
  Branches       6214     6214              
============================================
- Hits          44089    44085       -4     
+ Misses          253      252       -1     
- Partials        537      543       +6

Impacted Files	Coverage Δ	Complexity Δ
src/main/java/io/reactivex/Flowable.java	100% <100%> (ø)	566 <0> (ø)	⬇️
src/main/java/io/reactivex/Maybe.java	100% <100%> (ø)	171 <0> (ø)	⬇️
src/main/java/io/reactivex/Completable.java	100% <100%> (ø)	117 <0> (ø)	⬇️
src/main/java/io/reactivex/Observable.java	100% <100%> (ø)	541 <0> (ø)	⬇️
src/main/java/io/reactivex/Single.java	100% <100%> (ø)	146 <0> (ø)	⬇️
.../operators/flowable/FlowableBlockingSubscribe.java	93.02% <0%> (-4.66%)	10% <0%> (-1%)
...ternal/operators/observable/ObservablePublish.java	96.46% <0%> (-3.54%)	11% <0%> (ø)
.../io/reactivex/internal/schedulers/IoScheduler.java	89.24% <0%> (-2.16%)	9% <0%> (ø)
...nternal/operators/observable/ObservableCreate.java	97.43% <0%> (-1.71%)	2% <0%> (ø)
...x/internal/operators/flowable/FlowablePublish.java	97.87% <0%> (-1.71%)	11% <0%> (ø)
... and 25 more

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