2.x: Add explanation text to Undeliverable & OnErrorNotImplemented exs

[akarnokd]

This PR adds detailed error message to the UndeliverableException and OnErrorNotImplementedException as they are the most common exception-related questions around.

UndeliverableException

The exception could not be delivered to the consumer because it has already canceled/disposed the flow or the exception has nowhere to go to begin with. Further reading: https://github.com/ReactiveX/RxJava/wiki/What's-different-in-2.0#error-handling | <original exception message>

OnErrorNotImplementedException

The onError handler in the subscribe() method is not provided. Please specify it to avoid this type of exception. Further reading: https://github.com/ReactiveX/RxJava/wiki/Error-Handling | <original exception message>

[akarnokd]

/cc @artem-zinnatullin @davidmoten

[codecov]

Codecov Report

Merging #6171 into 2.x will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff              @@
##                2.x    #6171      +/-   ##
============================================
+ Coverage     98.21%   98.21%   +<.01%     
+ Complexity     6199     6197       -2     
============================================
  Files           667      667              
  Lines         44879    44879              
  Branches       6214     6214              
============================================
+ Hits          44076    44078       +2     
+ Misses          258      257       -1     
+ Partials        545      544       -1

Impacted Files	Coverage Δ	Complexity Δ
...vex/exceptions/OnErrorNotImplementedException.java	100% <100%> (ø)	4 <2> (-1)	⬇️
...o/reactivex/exceptions/UndeliverableException.java	100% <100%> (ø)	1 <1> (ø)	⬇️
...ternal/operators/observable/ObservablePublish.java	96.46% <0%> (-3.54%)	11% <0%> (ø)
...ernal/operators/flowable/FlowableFlatMapMaybe.java	90.82% <0%> (-3.39%)	2% <0%> (ø)
...activex/internal/observers/QueueDrainObserver.java	97.43% <0%> (-2.57%)	21% <0%> (-1%)
...va/io/reactivex/internal/queue/SpscArrayQueue.java	97.61% <0%> (-2.39%)	22% <0%> (-1%)
.../internal/disposables/ListCompositeDisposable.java	98% <0%> (-2%)	34% <0%> (-1%)
...internal/operators/flowable/FlowableSwitchMap.java	95.28% <0%> (-1.42%)	3% <0%> (ø)
...ava/io/reactivex/processors/BehaviorProcessor.java	95.96% <0%> (-0.9%)	59% <0%> (-1%)
.../operators/mixed/FlowableConcatMapCompletable.java	99.14% <0%> (-0.86%)	2% <0%> (ø)
... and 21 more

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[artem-zinnatullin]

Please specify it to avoid this type of exception.

I'm afraid this advice is too similar to generic error handling like this

try {
  method();
} catch (Throwable t) {
  t.printStackTrace();
}

What about something like:

An error occurred in the flow but no error handler was provided. Further reading: https://github.com/ReactiveX/RxJava/blob/2.x/docs/Error-Handling.md

[akarnokd]

"no error handler was provided"

I'm worried users will think having or adding doOnError will solve the error, hence the explicit mention of subscribe().

[artem-zinnatullin]

Oh, I see

Well, I'm not against mentioning subscribe() as long as we don't just say to "set this error handler to avoid this exception"

We can also mention onErrorXXX, might be a bit overwhelming tho

[akarnokd]

How about this:

The exception was not handled due to missing onError handler in the subscribe() method call.

[artem-zinnatullin]

👍