[JENKINS-34710] - PluginWrapper should not throw IOException if somebody enables the enabled plugin (#2327)

core/src/main/java/hudson/PluginWrapper.java

@@ -441,6 +441,10 @@ public class PluginWrapper implements Comparable<PluginWrapper>, ModelObject {
      * Enables this plugin next time Jenkins runs.
      */
     public void enable() throws IOException {
+        if (!disableFile.exists()) {
+            LOGGER.log(Level.FINEST, "Plugin {0} has been already enabled. Skipping the enable() operation", getShortName());
+            return;
+        }
         if(!disableFile.delete())
             throw new IOException("Failed to delete "+disableFile);
     }