Surface JUnit parse errors

This makes it easier for users to troubleshoot why the JUnit parsing failed:

The test_report.json endpoint previously just showed:

{
  "status": "error_parsing_report"
}

Now, it will show something like:

{
  "status": "error_parsing_report",
  "details": "XML parsing failed: 1:1: FATAL: Document is empty"
}

Closes #210328 (closed)