fix(cli): warn on unknown commands

[malept]

• I have read the contribution documentation for this project.
• I agree to follow the code of conduct that this project follows, as appropriate.
• The changes have sufficient test coverage (if applicable).
• The testsuite passes successfully on my local machine (if applicable).

Summarize your changes:

Emit a warning and exit with status code 1.

Based on tj/commander.js#1088 (comment)

Example:

$ electron-forge foobar foo
✔ Checking your system

Unknown command "foobar foo".
See --help for a list of available commands.

Fixes #1300
Closes #1301 (This is a simpler version, with an integration test added.)

[codecov]

Codecov Report

Merging #1304 into master will decrease coverage by 1.15%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1304      +/-   ##
==========================================
- Coverage   68.43%   67.28%   -1.16%     
==========================================
  Files          67       69       +2     
  Lines        1958     2002      +44     
  Branches      378      383       +5     
==========================================
+ Hits         1340     1347       +7     
- Misses        444      479      +35     
- Partials      174      176       +2

Impacted Files	Coverage Δ
packages/api/cli/src/electron-forge.ts	55.17% <100%> (ø)
packages/maker/flatpak/src/MakerFlatpak.ts	72.72% <0%> (-9.1%)	⬇️
packages/maker/zip/src/MakerZIP.ts	90.9% <0%> (-9.1%)	⬇️
packages/api/core/src/util/electron-version.ts	88.23% <0%> (-7.85%)	⬇️
packages/api/core/src/api/make.ts	64.77% <0%> (-5.69%)	⬇️
packages/maker/dmg/src/MakerDMG.ts	94.44% <0%> (-5.56%)	⬇️
packages/utils/async-ora/src/ora.ts	56.66% <0%> (-3.34%)	⬇️
packages/maker/base/src/Maker.ts	64.51% <0%> (-3.23%)	⬇️
packages/api/cli/src/util/terminate.ts	20% <0%> (ø)
... and 1 more

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