better logging of failure to load sigar under monitor.sigar, and not just monitor

src/main/java/org/elasticsearch/monitor/MonitorModule.java

@@ -89,7 +89,7 @@ public class MonitorModule extends AbstractModule {
             }
         } catch (Throwable e) {
             // no sigar
-            Loggers.getLogger(MonitorModule.class).trace("failed to load sigar", e);
+            Loggers.getLogger(SigarService.class).trace("failed to load sigar", e);
         }
         if (!sigarLoaded) {
             // bind non sigar implementations