chore(generators): send warning regarding invalid template to stderr

[jamesgeorge007]

What kind of change does this PR introduce?
chore

Did you add tests for your changes?
Update test suite.

If relevant, did you update the documentation?
N/A

Summary

webpack-cli/packages/generators/src/addon-generator.ts

Line 60 in af6c02b

this.utils.logger.warn(`⚠ ${this.template} is not a valid template, please select one from below`);

Does this PR introduce a breaking change?
Nope

Other information
N/A

[codecov]

Codecov Report

Merging #2687 (99e87ff) into master (af6c02b) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2687      +/-   ##
==========================================
- Coverage   96.06%   96.00%   -0.07%     
==========================================
  Files          30       30              
  Lines        1526     1526              
  Branches      432      432              
==========================================
- Hits         1466     1465       -1     
- Misses         60       61       +1     

Impacted Files	Coverage Δ
packages/generators/src/init-generator.ts	98.07% <100.00%> (ø)
packages/generators/src/addon-generator.ts	91.22% <0.00%> (-1.76%)	⬇️

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[ghost]

Lots of changes without explicit approval. Thanks.