feat(pkg/commands): provide ExitCodeError to use in place of os.Exit

[thehowl]

This PR substitutes uses of os.Exit(1), and the hack of --set-exit-status=0 used in tests for gno lint, so that the idiomatic way to return an error without printing anything becomes commands.ExitCodeError. Additionally, this can support returning different exit codes (other than 1).

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests
• Added new benchmarks to generated graphs, if any. More info here.

[codecov]

Codecov Report

Attention: Patch coverage is 16.66667% with 10 lines in your changes are missing coverage. Please review.

Project coverage is 49.91%. Comparing base (e8b26ec) to head (f2223c7).

Files	Patch %	Lines
tm2/pkg/commands/command.go	0.00%	6 Missing ⚠️
tm2/pkg/commands/errors.go	0.00%	2 Missing ⚠️
gnovm/cmd/gno/run.go	0.00%	1 Missing ⚠️
gnovm/cmd/gno/test.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2187      +/-   ##
==========================================
- Coverage   49.94%   49.91%   -0.03%     
==========================================
  Files         577      578       +1     
  Lines       77859    77707     -152     
==========================================
- Hits        38890    38791      -99     
+ Misses      35840    35804      -36     
+ Partials     3129     3112      -17     

Flag	Coverage Δ
gnovm	45.08% <50.00%> (-0.02%)	⬇️
tm2	54.38% <0.00%> (-0.11%)	⬇️

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[gfanton]

Nice idea !