fix: do not clean directory if webpack errors are present during initial build

[chrisblossom]

Please hold off on merging this.

Although this is a fix for #133, is it also a breaking change? I think it could be if someone is using a webpack plugin that manually creates files inside webpack's output.path after the compile hook, although this is not supported by webpack and probably can/should be ignored. Thoughts?

[codecov-io]

Codecov Report

Merging #134 into master will increase coverage by 0.09%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #134      +/-   ##
=========================================
+ Coverage   98.71%   98.8%   +0.09%     
=========================================
  Files           1       1              
  Lines          78      84       +6     
  Branches       27      28       +1     
=========================================
+ Hits           77      83       +6     
  Misses          1       1

Impacted Files	Coverage Δ
src/clean-webpack-plugin.ts	98.8% <100%> (+0.09%)	⬆️

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[thomastay]

Hi @chrisblossom @johnagan this PR broke our build :(, would appreciate feedback on how to upgrade to the new version.
We currently have two Webpack configs which emit different files into our shared dist/ folder. Since Webpack runs the configs in parallel, there can be a problem where one config cleans the other's emitted files.

With this change, whichever config finishes compiling second will clean up the other config's emitted files. Previously, since we cleaned before compiling, this didn't happen because both the cleans happened immediately, before any files were compiled. (technically, it was a race condition back then, but our compiles are long enough that it never surfaced)

Is there a way to only clean the files that would be emitted by webpack? Or any other thoughts?

Versions:
Webpack 4.44