2.x: add tryOnError to create/XEmitter API

[akarnokd]

This PR adds the tryOnError method to the various Emitter types used in the create operators that allows the developer to avoid the UndeliverableException in case a cancellation is racing with the emission of an error. The return value indicates a success of the delivery; in case of false, the developer can decide to log/drop the specific error if that makes sense for him/her.

[codecov]

Codecov Report

Merging #5344 into 2.x will increase coverage by 0.1%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##                2.x    #5344     +/-   ##
===========================================
+ Coverage     96.08%   96.18%   +0.1%     
- Complexity     5770     5782     +12     
===========================================
  Files           630      630             
  Lines         41146    41162     +16     
  Branches       5714     5721      +7     
===========================================
+ Hits          39536    39593     +57     
+ Misses          633      613     -20     
+ Partials        977      956     -21

Impacted Files	Coverage Δ	Complexity Δ
...eactivex/internal/operators/maybe/MaybeCreate.java	92% <100%> (+0.33%)	2 <0> (ø)	⬇️
...ernal/operators/completable/CompletableCreate.java	95.12% <100%> (+2.81%)	2 <0> (ø)	⬇️
...nternal/operators/observable/ObservableCreate.java	98.29% <100%> (-0.82%)	2 <0> (ø)
...ex/internal/operators/flowable/FlowableCreate.java	96.77% <100%> (-0.94%)	6 <0> (ø)
...ctivex/internal/operators/single/SingleCreate.java	95.34% <100%> (+0.22%)	2 <0> (ø)	⬇️
...vex/internal/operators/single/SingleTakeUntil.java	86.88% <0%> (-6.56%)	2% <0%> (ø)
...in/java/io/reactivex/subjects/BehaviorSubject.java	85.93% <0%> (-5.73%)	56% <0%> (ø)
.../operators/flowable/FlowableBlockingSubscribe.java	91.66% <0%> (-5.56%)	9% <0%> (-1%)
...internal/operators/completable/CompletableAmb.java	94.91% <0%> (-3.39%)	10% <0%> (-1%)
...x/internal/operators/maybe/MaybeSwitchIfEmpty.java	97.22% <0%> (-2.78%)	2% <0%> (ø)
... and 46 more

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[vanniktech]

indentation seems off

[artem-zinnatullin]

👍