improve error messages

[casperdcl]

The following have recently been documented in https://cml.dev/doc, but should also have better CLI errors pointing to said docs:

• cml send-comment --update: Raises 'commit_id has been locked' error #888
  • https://cml.dev/doc/ref/send-comment#GitHub
• send-comment PR for commit sha \"HEAD\" not found #880
  • uses: actions/checkout@v3, with.ref: ${{ github.event.pull_request.head.sha }}
  • https://cml.dev/doc/usage?tab=GitHub
• cml send-github-check hangs and does not report #941
  • use default GITHUB_TOKEN or GitHub App token, not Personal Access Token
  • https://cml.dev/doc/ref/send-github-check
• Error "Resource not accessible by integration" is not helpful #595
  • Resource not accessible by integration
  • Error: Forbidden
    • "Ensure permissions are granted as per https://cml.dev/doc/self-hosted-runners?tab=GitLab#personal-access-token"?

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TODO: merge this issue with #606?

[0x2b3bfa0]

Related to #606?

[casperdcl]

Indeed, can we collect actionable items in one place? Don't mind where.

[casperdcl]

closing in favour of #606