Track custom node startup errors and expose via API endpoint

[Kosinkadink]

Summary

Store import and prestartup errors for custom nodes and expose them via a new API endpoint, so the frontend/Manager can distinguish between a node pack that failed to import vs. one that is simply not installed.

Changes

• nodes.py - Added NODE_STARTUP_ERRORS dict; populated on import failure in load_custom_node()
• main.py - Populated NODE_STARTUP_ERRORS on prestartup script failure in execute_prestartup_script()
• server.py - Added GET /custom_node_startup_errors endpoint returning the error dict as JSON

API

GET /custom_node_startup_errors

// Response (empty when no errors)
{}

// Response (with errors)
{
  "my_custom_node": {
    "module_path": "path/to/my_custom_node",
    "error": "No module named 'some_dependency'",
    "traceback": "Traceback (most recent call last):...",
    "phase": "import"
  }
}

Ref: Comfy-Org/ComfyUI-Desktop-2.0-Beta#303