Save precondition errors on metadata

[jonesbusy]

Allow very outdated plugin that we cannot even build with recent Maven or Java 8 to have their metadata saved

Testing done

Tested with outdated plugin

Examples

{"errors":["MAVEN_REPOSITORIES_HTTP","MISSING_RELATIVE_PATH","OLDER_JAVA_VERSION"],"key":"plugin-metadata","path":"adaptive-disconnector"}

{"errors":["OLDER_JAVA_VERSION"],"key":"plugin-metadata","path":"vagrant"}

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[jonesbusy]

Ready for review. I've rebased the PR and fixed conflicts

[sridamul]

Thank you very much :-)