Allow exit failure only on specified severities

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Summary

Allow golangci-lint runs to exit with failed statuses only on specified issue severities

Fixes #1981

Details

• Define severity.fail-on-severities string slice config option as well as --fail-on-severities string slice command
  line option
• When this option is empty or nil (default value), keep behavior identical to current behavior
• When this option is not empty, only exit with an "issues found" code when at least one issue's severity matches one of
  values specified in the option

Notes / Concerns

• This implementation requires an explicit list of all codes resulting in failures; there is no "minimum cutoff" logic
  e.g. fail on warnings and above. A more sophisticated solution would need to establish total orderings of error
  severities across each output format and possibly across output formats, which is annoying to maintain and may
  confuse users.
• An empty config list is more strict than nonempty config lists, but larger config lists are more strict than smaller
  nonempty config lists; this is not that hard to fathom for users, since never exiting with a failed code is not too
  useful.
• Mixing config files with command line flags will merge the lists of exit codes upon which to fail; this can be a bit
  counterintuitive for the above reason.

Testing

• Add special test cases for the following cases
  • Info and warning issues with a config that fails on errors will result in a successful exit code
  • Info and warning issues with a config that fails on errors and warnings will result in a failed exit code
  • Info and warning issues with a config that fails on errors + a command line flag that fails on errors and
    warnings will fail

[boring-cyborg]

Hey, thank you for opening your first Pull Request !

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All committers have signed the CLA.

[ldez]

#2595 (comment)

#3184 (comment)

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#2595 (comment)

#3184 (comment)

It seems I misread previous discussions. That's a shame. Now we'll need multiple configurations to continue using the golangci-lint action or switch to a different action. We can't highlight deprecated code in editors like VSCode without breaking builds on deprecated code otherwise.