fix error message, add "appid" to the output to know which app caused the...

fix error message, add "appid" to the output to know which app caused the error if OC_App::getStorage() fails.
Port of approved patch for stable45: https://github.com/owncloud/core/pull/1222

lib/app.php

@@ -748,7 +748,7 @@ class OC_App{
 				}
 				return new OC_FilesystemView('/'.OC_User::getUser().'/'.$appid);
 			}else{
-				OC_Log::write('core', 'Can\'t get app storage, app, user not logged in', OC_Log::ERROR);
+				OC_Log::write('core', 'Can\'t get app storage, app '.$appid.', user not logged in', OC_Log::ERROR);
 				return false;
 			}
 		}else{