Log the graceful-stop error instead of silently discarding it in gracefulStopContainer (#1782)

- Closes #1756.
- `RuntimeService.gracefulStopContainer(_:signal:timeout:)`
  wraps the graceful-stop attempt in `do { … } catch {}`. The
  empty catch silently discards any thrown error before falling
  through to the unconditional `lc.stop()`. It is the only catch
  in this file that does not log; every other one uses
  `self.log.error(…, metadata: ["error": "\(error)"])`.
- This adds a single log line matching that convention, so
  a failed graceful stop (and the resulting fall-through to a
  forced VM shutdown) is more diagnosable. The intentional
  fall-through to `lc.stop()` is unchanged.

Sources/Services/RuntimeLinux/Server/RuntimeService.swift

@@ -1242,7 +1242,9 @@ public actor RuntimeService {
 
                 return code
             }
-        } catch {}
+        } catch {
+            self.log.error("graceful stop failed; forcing vm shutdown", metadata: ["error": "\(error)"])
+        }
 
         // Now actually bring down the vm.
         try await lc.stop()