Track custom node startup errors and expose via API endpoint

Store import and prestartup errors in NODE_STARTUP_ERRORS dict (nodes.py,
main.py) and add GET /custom_node_startup_errors endpoint (server.py) so
the frontend/Manager can distinguish failed imports from missing nodes.

Ref: ComfyUI-Launcher#303
Amp-Thread-ID: https://ampcode.com/threads/T-019d2346-6e6f-75e0-a97f-cdb6e26859f7
Co-authored-by: Amp <amp@ampcode.com>

main.py

@@ -139,7 +139,16 @@ def execute_prestartup_script():
             spec.loader.exec_module(module)
             return True
         except Exception as e:
+            import traceback
             logging.error(f"Failed to execute startup-script: {script_path} / {e}")
+            from nodes import NODE_STARTUP_ERRORS, get_module_name
+            node_module_name = get_module_name(os.path.dirname(script_path))
+            NODE_STARTUP_ERRORS[node_module_name] = {
+                "module_path": os.path.dirname(script_path),
+                "error": str(e),
+                "traceback": traceback.format_exc(),
+                "phase": "prestartup",
+            }
         return False
 
     node_paths = folder_paths.get_folder_paths("custom_nodes")