audit: round three on the port — the hardware scan check fails soft

A rig hiccup mid-walk records the scan check as failed and lets the suite
continue, matching its siblings' envelope; a nonexistent --port path
reports as an error instead of a traceback.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pureboot/pureboot.py

@@ -1600,7 +1600,7 @@ def main():
 if __name__ == "__main__":
     try:
         main()
-    except Error as error:
+    except (Error, OSError) as error:
         print(f"error: {error}", file=sys.stderr)
         sys.exit(1)
     except KeyboardInterrupt:

tools/pbhw.py

@@ -82,6 +82,7 @@ class Suite:
         on silicon."""
         module = pbrig.load_pureboot(self.rig.d.pureboot)
         found = None
+        try:
             for pct in module.scan_ratios():
                 rate = module.scan_rate(self.rig.d.baud, pct)
                 self.rig.reset()
@@ -98,6 +99,9 @@ class Suite:
                     continue
                 finally:
                     port.close()
+        except Exception as error:  # noqa: BLE001 — a rig hiccup is a result
+            self.check("scan walks the probe ladder", False, str(error)[:70])
+            return
         self.check("scan finds the board's rate", found is not None,
                    "no probe answered" if found is None else f"{found:+d} % of {self.rig.d.baud} Bd")