fix: Show custom node import failure reasons in summary

[RUiNtheExtinct]

Summary

Improves custom node import failure diagnostics by showing the exception type and message in the import summary.

Before:

Import times for custom nodes:
   0.0 seconds (IMPORT FAILED): custom_nodes/my_node

After:

Import times for custom nodes:
   0.0 seconds (IMPORT FAILED: ImportError: No module named 'xyz'): custom_nodes/my_node

Problem

When custom nodes fail to import, users only see (IMPORT FAILED) in the summary. To understand why, they must scroll up through the logs to find the traceback. This is especially frustrating when multiple nodes fail.

Reported in #11454.

Changes

• Add IMPORT_FAILED_REASONS module-level dict to store failure context
• Capture exception type and first line of message (truncated to 100 chars)
• Include failure reason in the import summary output
• Full traceback is still logged separately for detailed debugging

Testing

• Added unit tests covering:
  • Correct formatting of various exception types (ImportError, SyntaxError, etc.)
  • Truncation of long error messages
  • Multiline error handling (only first line used)
  • Summary output format

Compatibility

• Backward compatible - no breaking changes
• Existing behavior preserved (full traceback still logged)
• No new dependencies

Fixes #11454