[master] Make host file helpers report failure instead of silently patching

[joaoantoniocardoso]

Port of #4242 into master.

Fix #4235.

[github-actions]

Automated PR Review

0. Summary

• Verdict: MINOR SUGGESTIONS ✏️

Introduces HostFileError, makes load_file/save_file raise on failure instead of silently returning empty strings, fixes locate_file to actually return None (matching its Optional[str] annotation), updates the three callers (get_host_os, NavigatorPi4.get_serials, blueos_startup_update) to handle the new exception, and wraps the patch-application loop in a per-patch try/except so one bad patch no longer aborts the startup. Adds pytest coverage for the new behavior.

1. Correctness & Implementation Bugs

• 1.1 [minor] core/tools/blueos_startup_update/blueos_startup_update.py:822 — the new per-patch guard except Exception as patch_error: logger.error(f"Patch {name} failed: {patch_error}") swallows the traceback. Because the caller is now the last line of defense against a broken patch (previously the process would die and be visible in logs), the stack trace is exactly what an operator will want when triaging a boot failure. Prefer logger.exception(f"Patch {name} failed") (loguru supports it) so the traceback is captured.

6. Code Quality & Style

• 6.1 [nit] core/libs/commonwealth/src/commonwealth/utils/commands.py:151 — save_file now silently ignores a failed backup (logger.warning only) but raises on a failed write. That is likely intentional (missing source file → nothing to back up), but the asymmetry is worth a one-line comment so future readers do not "fix" it by promoting the warning to a raise.
• 6.2 [nit] core/tools/blueos_startup_update/blueos_startup_update.py:295-297,320-322 — the new guards use logging.warning(...) (stdlib) while the rest of these functions use logger (loguru). This matches the pre-existing update_cgroups/update_dwc2/update_navigator_overlays guards, so the inconsistency is not introduced here — but it would be a good moment to unify on logger for the file. Optional.

7. Tests

• 7.1 [nit] core/libs/commonwealth/src/commonwealth/utils/tests/test_commands.py:60 — test_save_file_raises_when_upload_fails monkeypatches run_command to always return returncode=0, which means the backup step "succeeds" but upload_file is mocked separately. Consider one more case where the backup cp fails (returncode!=0) but upload_file succeeds, to lock in the "log warning, do not raise" behavior added in save_file.
• 7.2 [nit] No test covers the new try/except wrapper around the patch loop in blueos_startup_update.main. A tiny test that stubs two patches (one raising, one returning True) and asserts the good one still ends up in patches_requiring_restart would guard the isolation property that motivates this PR.

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