2.x: FlowableScan - prevent multiple terminal emissions

[davidmoten]

For scan without seed this PR

• prevents multiple terminal events being emitted when the scan function throws
• prevents processing of a later onNext if the previous onNext processing resulted in an error emission
• ensures post terminal errors are reported to RxJavaPlugins error handler

For scan with seed (forgot this one in the last PR)

• ensures post terminal errors are reported to RxJavaPlugins error handler

[codecov-io]

Current coverage is 95.68% (diff: 100%)

Merging #4901 into 2.x will increase coverage by 0.01%

@@                2.x      #4901   diff @@
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+ Misses          678        677     -1   
+ Partials        942        937     -5   

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