All plugins can optionally add an init function named `pluginname_init()` which is called when the plugin is loaded.
This function is aware of the config, and can return initialization errors, which are displayed in the header template.
Note that the previous error system hack no longer work.
Before this, calling writeConfig() would have write error messages in data.php, because it uses 'plugins' array which is used for plugin configuration.
Causing the message error appear everytime.
