Add license key error to troubleshooting page

[IanMadd]

Description

InSpec 6 requires a license key and if it's not provided it will give an error.

Add text in the troubleshooting page that describes the error and links to docs for correcting the error.

Related Issue

#6574

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New content (non-breaking change)
• Breaking change (a content change which would break existing functionality or processes)

Checklist:

• I have read the CONTRIBUTING document.

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[chef-expeditor]

Hello IanMadd! Thanks for the pull request!

Here is what will happen next:

1. Your PR will be reviewed by the maintainers.
2. Possible Outcomes
   a. If everything looks good, one of them will approve it, and your PR will be merged.
   b. The maintainer may request follow-on work (e.g. code fix, linting, etc). We would encourage you to address this work in 2-3 business days to keep the conversation going and to get your contribution in sooner.
   c. Cases exist where a PR is neither aligned to Chef InSpec's product roadmap, or something the team can own or maintain long-term. In these cases, the maintainer will provide justification and close out the PR.

Thank you for contributing!

[clintoncwolfe]

Might be clearer to say "Exit Code 173" - we don't have numbered error messages, and this somewhat implies that we do.

[Nik08]

It is exit code 174 when license is not set for inspec
Exit code 173 is when license is not entitled to inspec software