3.x: Cleanup addThrowable, "2.x" and null-value error messages

[akarnokd]

This PR renames the internal addThrowable to tryAddThrowable and introduces the tryAddThrowableOrReport to perform the common reporting to the global error handler. Usage places have been re-evaluated and fixed if necessary.

In addition, null exceptions have been made more uniform.

Resolves #6611

[codecov]

Codecov Report

Merging #6639 into 3.x will increase coverage by 0.01%.
The diff coverage is 98.73%.

@@             Coverage Diff              @@
##                3.x    #6639      +/-   ##
============================================
+ Coverage     98.16%   98.17%   +0.01%     
- Complexity     6180     6193      +13     
============================================
  Files           678      678              
  Lines         44784    44690      -94     
  Branches       6177     6165      -12     
============================================
- Hits          43960    43876      -84     
+ Misses          282      279       -3     
+ Partials        542      535       -7

Impacted Files	Coverage Δ	Complexity Δ
...nal/operators/flowable/FlowableWindowBoundary.java	99.26% <100%> (-0.02%)	2 <0> (ø)
...3/internal/operators/flowable/FlowableGroupBy.java	95.51% <100%> (ø)	3 <0> (ø)	⬇️
...s/completable/CompletableMergeDelayErrorArray.java	97.61% <100%> (-0.06%)	6 <0> (ø)
...l/operators/observable/ObservableToListSingle.java	100% <100%> (ø)	4 <1> (ø)	⬇️
...internal/operators/observable/ObservableError.java	100% <100%> (ø)	2 <1> (ø)	⬇️
...nal/operators/flowable/FlowableBufferBoundary.java	95.95% <100%> (-0.07%)	2 <0> (ø)
...perators/mixed/ObservableSwitchMapCompletable.java	100% <100%> (ø)	3 <0> (ø)	⬇️
...ernal/operators/flowable/FlowableFlatMapMaybe.java	92.7% <100%> (-3.69%)	2 <0> (ø)
.../operators/observable/ObservableFlatMapSingle.java	96.03% <100%> (+4.62%)	2 <0> (ø)	⬇️
...rxjava3/internal/operators/single/SingleError.java	100% <100%> (ø)	2 <1> (ø)	⬇️
... and 104 more

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