fix: log which import failed when resolving another group's output

[amannocci]

🧑‍💻What is the change being made?

When a manifest imports entry cannot be read from another resource group, terranova now logs a failure that names the import, its source group and the importing group, followed by a cause and resolution. This adds an ImportOutputError and logs it in extract_output_var before exiting with the same exit code as before. Self-imports still raise SelfImportNotReadyError.

❓ Why is the change being made?

A failing import used to exit silently with only terraform's raw output stderr, so users could not tell whether the import or the resource group's own plan/apply had failed.

✅ How has this been tested?

Added test_error_logs_failing_import, which checks that stderr names the import, its source and the importing group, and includes a cause. The tests/it suite passes locally (368 tests), along with ruff and basedpyright.

🔗 Related Issues

Closes #97