Make sure headers are not set twice on error

[zbynek]

Handle the case when headers are already sent because of an error -- we want to report the error, not hide it behind "headers already sent". Also bump Node requirement to match versions not affected by #756 .

Relevant issues

#756

Contributor checklist

• Provide tests for the changes (unless documentation-only)
• Documented any new features, CLI switches, etc. (if applicable)
  • Server --help output
  • README.md
  • doc/http-server.1 (use the same format as other entries)
• The pull request is being made against the master branch

Maintainer checklist

• Assign a version triage tag
• Approve tests if applicable

[zbynek]

@thornjad ping