Avoid hanging processes in the lapply-like backends

[wlandau]

Summary

Previously, there were problems with hanging processes in the mclapply and parLapply backends when one of the processes exited in error. In this PR, I use on.exit() to enforce the following.

1. If the master dies, all the workers die.
2. If all the workers die, the master dies.

Also, the error log of the process is cached in the "mc_fail" storr namespace. For example, retrieve with config$cache$get("2", namespace = "mc_fail") for worker 2 and config$cache$get("0", namespace = "mc_fail") for master. I haven't wrote a friendly API function to retrieve these types of errors, and I currently do not have plans for one. This error caching is mostly just to help me and the contributors debug. Will revisit if these errors come up a lot, but I am expecting most errors to be related to targets once #369 is solved, and there is already error handling for that.

Currently affects mclapply and parLapply parallelism, and will affect future_lapply parallelism going forward (hopefully soon).

GitHub issues addressed

• Ref: A quicker end to staged parallelism #369 (not done yet)

Checklist

• I have read drake's code of conduct, and I agree to follow its rules.
• I have read the guidelines for contributing.
• I have listed any substantial changes in the development news.
• I have added testthat unit tests to tests/testthat to confirm that any new features or functionality work correctly.
• I have tested this pull request locally with devtools::check()
• This pull request is ready for review.
• I think this pull request is ready to merge.

[codecov-io]

Codecov Report

Merging #373 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@           Coverage Diff            @@
##           master   #373      +/-   ##
========================================
+ Coverage    99.9%   100%   +0.09%     
========================================
  Files          66     66              
  Lines        5467   5485      +18     
========================================
+ Hits         5462   5485      +23     
+ Misses          5      0       -5

Impacted Files	Coverage Δ
R/handlers.R	100% <ø> (ø)	⬆️
R/mclapply.R	100% <100%> (ø)	⬆️
R/migrate.R	100% <0%> (+2.15%)	⬆️

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