Fix Checkstyle warnings about empty catch blocks

Most of them are empty because the exceptions are ignored.  In these
cases add an "Ignored" comment to prevent the warning.

In the other couple of cases the problem might be worth investigation
by an admin, so add a warning log.

Change-Id: I415fa042f5205cb0745ef7286fba5f5910f1dedd

gerrit-server/src/main/java/com/google/gerrit/server/plugins/PluginCleanerTask.java

@@ -45,6 +45,7 @@ class PluginCleanerTask implements Runnable {
         Thread.sleep(50);
       }
     } catch (InterruptedException e) {
+      // Ignored
     }
 
     int left = loader.processPendingCleanups();

gerrit-server/src/main/java/com/google/gerrit/server/plugins/PluginScannerThread.java

@@ -37,6 +37,7 @@ class PluginScannerThread extends Thread {
           return;
         }
       } catch (InterruptedException e) {
+        // Ignored
       }
       loader.rescan();
     }
@@ -47,6 +48,7 @@ class PluginScannerThread extends Thread {
     try {
       join();
     } catch (InterruptedException e) {
+      // Ignored
     }
   }
 }