Fix asynchronous Python exception propagation in StreamingPythonExecutor/CNNScoreVariants.

[cmnbroad]

Fix for #7401. Draft mode for now until I do more verification.

[gatk-bot]

Travis reported job failures from build 35396
Failures in the following jobs:

Test Type	JDK	Job ID	Logs
conda	openjdk8	35396.5	logs

[droazen]

@cmnbroad Is this still a draft, or is it ready for review?

[cmnbroad]

@doazen I need to re-review this myself, and see what more validation I can do. I hate to miss the release, but I won't be able to do that today.

[droazen]

@cmnbroad No problem, this isn't particularly time-sensitive. Ping me once it's ready for review.

[cmnbroad]

@droazen I thought I had updated this months ago saying that I was finished validating, and that it was ready for review, but it appears that I didn't. But anyway its ready.

[codecov]

Codecov Report

Merging #7402 (b61283e) into master (9ae1fd8) will increase coverage by 0.002%.
The diff coverage is 84.444%.

@@               Coverage Diff               @@
##              master     #7402       +/-   ##
===============================================
+ Coverage     86.954%   86.956%   +0.002%     
- Complexity     36897     36910       +13     
===============================================
  Files           2214      2214               
  Lines         173540    173578       +38     
  Branches       18736     18736               
===============================================
+ Hits          150900    150937       +37     
- Misses         16037     16042        +5     
+ Partials        6603      6599        -4     

Impacted Files	Coverage Δ
...bender/utils/runtime/AsynchronousStreamWriter.java	81.633% <0.000%> (-2.041%)	⬇️
.../walkers/vqsr/CNNScoreVariantsIntegrationTest.java	95.722% <77.778%> (-0.907%)	⬇️
.../python/StreamingPythonScriptExecutorUnitTest.java	86.957% <84.615%> (-0.763%)	⬇️
...tools/examples/ExampleStreamingPythonExecutor.java	97.222% <100.000%> (+0.556%)	⬆️
...er/utils/python/StreamingPythonScriptExecutor.java	85.345% <100.000%> (-0.125%)	⬇️
...nder/utils/io/DeleteRecursivelyOnExitPathHook.java	80.952% <0.000%> (-9.524%)	⬇️
...itute/hellbender/tools/LocalAssemblerUnitTest.java	92.448% <0.000%> (ø)
...ellbender/tools/walkers/vqsr/CNNScoreVariants.java	79.565% <0.000%> (+0.870%)	⬆️
...nder/utils/runtime/StreamingProcessController.java	69.091% <0.000%> (+1.818%)	⬆️
.../hellbender/utils/python/PythonUnitTestRunner.java	78.689% <0.000%> (+3.279%)	⬆️

[lbergelson]

This looks google to me.