Some fixes to avoid status error 500 when it should return 404

- Another thing to consider, the right enabling/disabling feature must be placed as following:

    -----

    class erikPlugin extends PMPlugin
    {

      ...

      public function enable()
      {
          $this->enableRestService(true);
      }

      public function disable()
      {
          $this->enableRestService(false);
      }

    }

workflow/engine/classes/class.pluginRegistry.php

@@ -1415,11 +1415,11 @@ class PMPluginRegistry
                         "filepath" => $classFile,
                         "namespace" => $ns
                     );
+                }
+            }
+        }
 
         \Maveriks\WebApplication::purgeRestApiCache(basename(PATH_DATA_SITE));
-                }
-            }
-        }
 
         return true;
     }
@@ -1432,6 +1432,7 @@ class PMPluginRegistry
     public function unregisterRestService ($sNamespace)
     {
         unset($this->_restServices[$sNamespace]);
+        \Maveriks\WebApplication::purgeRestApiCache(basename(PATH_DATA_SITE));
     }
 
     public function getRegisteredRestServices()