2.x: Report errors from onError to Plugin when done

[vanniktech]

For the first one that I found I even wrote a test. Then I started noticing more missing spots. If wanted I could port that one test to every operator that I touched. Though I didn't really find tests testing that errors are propagated through the Plugin API.

[codecov-io]

Current coverage is 78.14% (diff: 45.45%)

Merging #4590 into 2.x will increase coverage by 0.06%

@@                2.x      #4590   diff @@
==========================================
  Files           552        552          
  Lines         36220      36247    +27   
  Methods           0          0          
  Messages          0          0          
  Branches       5594       5594          
==========================================
+ Hits          28282      28326    +44   
+ Misses         5927       5914    -13   
+ Partials       2011       2007     -4   

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[akarnokd]

TestHelper has a few generic test methods to check Maybe and Single. The same pattern could be extended to test these without duplicating that long method. I was planning that as part of another run of coverage improvements but I'm behind with performance work too.