fix: improve error messages for plugin installation failures

[Barneyjm]

When OAuth authentication plugins fail to install (e.g., due to corporate npm proxy issues), users now receive:

• Clear explanation of which authentication method is unavailable
• Actionable troubleshooting steps (check network, verify .npmrc, etc.)
• Detailed error information for debugging

Also adds:

• Plugin.getFailedPlugins() function for programmatic access
• /plugin/failed API endpoint for diagnostic purposes

Fixes #8297

How did you verify your code works?

Built and set npm to a fake URL to verify the error messages