Polish the function get_plugin_path: return None when the plugin is not found.

modules/plugin_dbui/helper.py

@@ -64,7 +64,7 @@ def get_plugin_path(environment, plugin_name):
     The dictionary environment contains the keys request, response, 
     session, plugins, ....
     
-    Raise the exception HTTP(500) if the plugin is not found.
+    Return None if the plugin is not found.
     
     """
     server_path, client_path = get_reference_paths(environment)
@@ -77,7 +77,7 @@ def get_plugin_path(environment, plugin_name):
                 return os.path.join(client_path, 'static', el)
 
     # Nothing found
-    raise HTTP(500, 'The plugin %s is not install !' % plugin_name)
+    return None
 
     
 def get_js_files(server_path, client_path, directory):