fix: propagate errors raised when loading the webpack configuration file

[micalevisk]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
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PR Type

What kind of change does this PR introduce?

[x] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Other... Please describe:

What is the current behavior?

Issue Number: closes #2421

What is the new behavior?

propagate any errors raised when loading the webpack configuration file. For example:

1. when webpack config path is not supplied but the file do exists

in this case I didn't installed run-script-webpack-plugin and the config file is using it

2. when the supplied webpack config path is not found

Does this PR introduce a breaking change?

[ ] Yes
[x] No(?)

[kamilmysliwiec]

lgtm