fix: error message for missing default export in configuration

[alexander-akait]

What kind of change does this PR introduce?

bugfix

Did you add tests for your changes?

Yes

If relevant, did you update the documentation?

No need

Summary

fixes #3684

Does this PR introduce a breaking change?

No

Other information

No

[codecov]

Codecov Report

Merging #3685 (a408f54) into master (f642f8d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3685      +/-   ##
==========================================
+ Coverage   91.37%   91.38%   +0.01%     
==========================================
  Files          22       22              
  Lines        1588     1591       +3     
  Branches      447      448       +1     
==========================================
+ Hits         1451     1454       +3     
  Misses        137      137              

Impacted Files	Coverage Δ
packages/webpack-cli/src/webpack-cli.ts	92.73% <100.00%> (+0.02%)	⬆️

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