fix: catch worker-farm errors

[the-ress]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update (if you have an idea how to test this without hacks or major changes, let me know)
• typo fix
• metadata update

Motivation / Use-Case

When the worker process fails to spawn, (and only when using cache), the error turns into an unhandled promise rejection and uglifyjs-webpack-plugin hangs and never finishes.

This PR fixes that by catching the error and handling it the same way as in the non-concurrent path.

Unhandled rejection Error: spawn ENOMEM
    at ChildProcess.spawn (internal/child_process.js:313:11)
    at exports.spawn (child_process.js:503:9)
    at Object.exports.fork (child_process.js:104:10)
    at fork (node_modules/worker-farm/lib/fork.js:17:36)
    at Farm.startChild (node_modules/worker-farm/lib/farm.js:106:16)
    at Farm.processQueue (node_modules/worker-farm/lib/farm.js:279:10)
    at Farm.addCall (node_modules/worker-farm/lib/farm.js:307:8)
    at Farm.<anonymous> (node_modules/worker-farm/lib/farm.js:38:10)
    at _class.boundWorkers (node_modules/uglifyjs-webpack-plugin/dist/uglify/index.js:71:24)
    at enqueue (node_modules/uglifyjs-webpack-plugin/dist/uglify/index.js:96:17)

Breaking Changes

I don't think there are any.

Additional Info

[jsf-clabot]

All committers have signed the CLA.

[alexander-akait]

Thanks for PR, can you add tests and comment in code about problem (minimum)?

[the-ress]

How would you approach testing this?

[alexander-akait]

@the-ress you need mock workerFarm (example how we mock cachache https://github.com/webpack-contrib/uglifyjs-webpack-plugin/blob/master/test/cache-option.test.js#L73), throw error into mock, set parallel: true and snapshot error (https://github.com/webpack-contrib/uglifyjs-webpack-plugin/blob/master/test/cache-option.test.js#L80). It is allow to ensure what our code works as expected and allow testing this code on CI to avoid problem for other developers.

[codecov]

Codecov Report

Merging #372 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #372      +/-   ##
==========================================
+ Coverage   97.69%   97.71%   +0.02%     
==========================================
  Files           5        5              
  Lines         260      263       +3     
  Branches      102      102              
==========================================
+ Hits          254      257       +3     
  Misses          6        6

Impacted Files	Coverage Δ
src/TaskRunner.js	97.87% <100%> (+0.14%)	⬆️

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[the-ress]

@evilebottnawi I added the tests and comment.

[alexander-akait]

Looks very good, need move test in https://github.com/webpack-contrib/uglifyjs-webpack-plugin/blob/master/test/parallel-option.test.js (group tests together) and update snapshot for windows (use -u flag)

[the-ress]

@evilebottnawi Does the Windows build pass on master?

[alexander-akait]

@the-ress yes, sometimes appveyor have problem with caching npm/yarn on their side, need just update snapshot

[alexander-akait]

/cc @the-ress

[alexander-akait]

Close in favor #380, thanks for PR and sorry for delay (many works)