No error reported for specs that do not exist #64

$ python-inspector --spec "foobar-does-not-exists" --json -
{
  "headers": {
    "tool_name": "python-inspector",
    "tool_homepageurl": "https://github.com/nexB/python-inspector",
    "tool_version": "0.6.5",
    "options": [
      "--specifier foobar-does-not-exists",
      "--index-url https://pypi.org/simple",
      "--python-version 38",
      "--operating-system linux",
      "--json <file>"
    ],
    "notice": "Dependency tree generated with python-inspector.\npython-inspector is a free software tool from nexB Inc. and others.\nVisit https://github.com/nexB/scancode-toolkit/ for support and download.",
    "warnings": [],
    "errors": []
  },
  "requirements": [
    {
      "purl": "pkg:pypi/foobar-does-not-exists",
      "extracted_requirement": "foobar-does-not-exists",
      "scope": "install",
      "is_runtime": true,
      "is_optional": false,
      "is_resolved": false,
      "resolved_package": {},
      "extra_data": {}
    }
  ],
  "resolved_dependencies": null
}