Display more detailed error messages

[fujidaiti]

Maybe messages in the summary fields of pana's output would help.

   {
    "id": "convention",
    "title": "Follow Dart file conventions",
    "grantedPoints": 30,
    "maxPoints": 30,
    "status": "passed",
    "summary": "### [*] 10/10 points: Provide a valid `pubspec.yaml`\n\n\n### [*] 5/5 points: Provide a valid `README.md`\n\n\n### [*] 5/5 points: Provide a valid `CHANGELOG.md`\n\n\n### [*] 10/10 points: Use an OSI-approved license\n\nDetected license: `MIT`."
   },