Improve source map script

[koesie10]

This fixes some minor bugs in the source map script:

• It now shows which gh command failed instead of just showing a generic error which didn't include which command failed
• It now takes into account both the label and the name of the release assets, which can apparently be different:

  {
     "apiUrl": "https://api.github.com/repos/github/vscode-codeql/releases/assets/198176998",
     "contentType": "application/zip",
     "createdAt": "2024-10-10T12:38:02Z",
     "downloadCount": 4,
     "id": "RA_kwDODJYu-M4Lz_Dm",
     "label": "vscode-codeql-sourcemaps-v1.16.0.zip",
     "name": "vscode-codeql-sourcemaps.zip",
     "size": 3509607,
     "state": "uploaded",
     "updatedAt": "2024-10-10T12:38:02Z",
     "url": "https://github.com/github/vscode-codeql/releases/download/v1.16.0/vscode-codeql-sourcemaps.zip"
   }

• It downloads the release assets to the artifacts directory so they are gitignored.