fix: Show custom node import failure reasons in summary

When custom nodes fail to import, the summary now shows the exception
type and message instead of just "(IMPORT FAILED)".

Before:
  0.0 seconds (IMPORT FAILED): custom_nodes/my_node

After:
  0.0 seconds (IMPORT FAILED: ImportError: No module named 'xyz'): custom_nodes/my_node

Changes:
- Add IMPORT_FAILED_REASONS dict to store failure context
- Capture exception type and first line of message (max 100 chars)
- Include failure reason in import summary output

This helps users quickly diagnose why custom nodes failed to load
without needing to scroll through the full traceback.

Fixes #11454

nodes.py

@@ -2103,6 +2103,10 @@ EXTENSION_WEB_DIRS = {}
 # Dictionary of successfully loaded module names and associated directories.
 LOADED_MODULE_DIRS = {}
 
+# Dictionary of import failure reasons keyed by module path.
+# Used to provide diagnostic information in the import summary.
+IMPORT_FAILED_REASONS: dict[str, str] = {}
+
 
 def get_module_name(module_path: str) -> str:
     """
@@ -2217,6 +2221,9 @@ async def load_custom_node(module_path: str, ignore=set(), module_parent="custom
             logging.warning(f"Skip {module_path} module for custom nodes due to the lack of NODE_CLASS_MAPPINGS or NODES_LIST (need one).")
             return False
     except Exception as e:
+        # Capture one-line failure reason for the import summary
+        error_msg = str(e).split('\n')[0][:100]  # First line, max 100 chars
+        IMPORT_FAILED_REASONS[module_path] = f"{type(e).__name__}: {error_msg}"
         logging.warning(traceback.format_exc())
         logging.warning(f"Cannot import {module_path} module for custom nodes: {e}")
         return False
@@ -2262,7 +2269,12 @@ async def init_external_custom_nodes():
             if n[2]:
                 import_message = ""
             else:
-                import_message = " (IMPORT FAILED)"
+                # Include failure reason if available
+                reason = IMPORT_FAILED_REASONS.get(n[1], "")
+                if reason:
+                    import_message = f" (IMPORT FAILED: {reason})"
+                else:
+                    import_message = " (IMPORT FAILED)"
             logging.info("{:6.1f} seconds{}: {}".format(n[0], import_message, n[1]))
         logging.info("")
 

tests-unit/nodes_test/test_import_failure_reasons.py

@@ -0,0 +1,89 @@
+"""Tests for custom node import failure reason reporting."""
+
+import pytest
+import tempfile
+import os
+import shutil
+from unittest.mock import patch, MagicMock
+import asyncio
+
+
+class TestImportFailureReasons:
+    """Test that import failures include diagnostic information."""
+
+    def test_import_failure_reason_format(self):
+        """Test that failure reason is formatted correctly."""
+        # Simulate the formatting logic
+        exception = ImportError("No module named 'missing_dep'")
+        error_msg = str(exception).split('\n')[0][:100]
+        reason = f"{type(exception).__name__}: {error_msg}"
+
+        assert reason == "ImportError: No module named 'missing_dep'"
+
+    def test_import_failure_reason_truncation(self):
+        """Test that long error messages are truncated."""
+        long_msg = "a" * 200
+        exception = ValueError(long_msg)
+        error_msg = str(exception).split('\n')[0][:100]
+        reason = f"{type(exception).__name__}: {error_msg}"
+
+        # Should be truncated to 100 chars for the message part
+        assert len(error_msg) == 100
+        assert reason.startswith("ValueError: ")
+
+    def test_import_failure_reason_multiline(self):
+        """Test that only first line of error is used."""
+        multi_line_msg = "First line\nSecond line\nThird line"
+        exception = RuntimeError(multi_line_msg)
+        error_msg = str(exception).split('\n')[0][:100]
+        reason = f"{type(exception).__name__}: {error_msg}"
+
+        assert reason == "RuntimeError: First line"
+        assert "Second line" not in reason
+
+    def test_import_failure_reason_various_exceptions(self):
+        """Test formatting for various exception types."""
+        test_cases = [
+            (ModuleNotFoundError("No module named 'foo'"), "ModuleNotFoundError: No module named 'foo'"),
+            (SyntaxError("invalid syntax"), "SyntaxError: invalid syntax"),
+            (AttributeError("'NoneType' object has no attribute 'bar'"), "AttributeError: 'NoneType' object has no attribute 'bar'"),
+            (FileNotFoundError("[Errno 2] No such file"), "FileNotFoundError: [Errno 2] No such file"),
+        ]
+
+        for exception, expected in test_cases:
+            error_msg = str(exception).split('\n')[0][:100]
+            reason = f"{type(exception).__name__}: {error_msg}"
+            assert reason == expected, f"Failed for {type(exception).__name__}"
+
+
+class TestImportSummaryOutput:
+    """Test the import summary output format."""
+
+    def test_summary_message_with_reason(self):
+        """Test that summary includes reason when available."""
+        reason = "ImportError: No module named 'xyz'"
+        import_message = f" (IMPORT FAILED: {reason})"
+
+        assert import_message == " (IMPORT FAILED: ImportError: No module named 'xyz')"
+
+    def test_summary_message_without_reason(self):
+        """Test fallback when no reason is available."""
+        reason = ""
+        if reason:
+            import_message = f" (IMPORT FAILED: {reason})"
+        else:
+            import_message = " (IMPORT FAILED)"
+
+        assert import_message == " (IMPORT FAILED)"
+
+    def test_summary_format_string(self):
+        """Test the full summary line format."""
+        time_taken = 0.05
+        import_message = " (IMPORT FAILED: ImportError: missing module)"
+        module_path = "/path/to/custom_nodes/my_node"
+
+        summary_line = "{:6.1f} seconds{}: {}".format(time_taken, import_message, module_path)
+
+        assert "0.1 seconds" in summary_line
+        assert "(IMPORT FAILED: ImportError: missing module)" in summary_line
+        assert module_path in summary_line