Avoid this[MODULE_TYPE] is not a function error

Following webpack-contrib#73 (comment)

Author: MoOx

src/loader.js

@@ -124,6 +124,9 @@ export function pitch(request) {
           };
         });
       }
+      if(typeof this[MODULE_TYPE] !== "function") {
+        return callback(new Error("It seems you are using the loader but forgot to register the plugin"));
+      }
       this[MODULE_TYPE](text);
     } catch (e) {
       return callback(e);