Update BlockingFlowableIterable.onNext() to set error before cancel

[kaioni17]

To avoid race with hasNext(), which checks for cancel first before checking for error. For example, in the following case, hasNext() may return false to the caller, making the caller assume the iterable finished successfully.

1. onNext() called cancel
2. hasNext() found the iterable is cancelled
3. hasNext() found that error is null thus returned false to the caller, without throwing the error
4. onNext() set error

It's a bit hard to test this race condition in unit tests. Existing tests can make sure no regression is introduced.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.62%. Comparing base (9b55d01) to head (e526cc3).
Report is 112 commits behind head on 3.x.

Additional details and impacted files

@@            Coverage Diff            @@
##                3.x    #7789   +/-   ##
=========================================
  Coverage     99.62%   99.62%           
  Complexity     6801     6801           
=========================================
  Files           752      752           
  Lines         47707    47711    +4     
  Branches       6401     6402    +1     
=========================================
+ Hits          47527    47532    +5     
+ Misses           84       81    -3     
- Partials         96       98    +2     

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[akarnokd]

Thanks for catching this.

[kaioni17]

Thanks for the review!