Register a settings error to be displayed to the user
Part of the Settings API. Use this to show messages to users about settings validation problems, missing settings or anything else.
Settings errors should be added inside the $sanitize_callback function defined in register_setting() for a given setting to give feedback about the submission.
By default messages will show immediately after the submission that generated the error. Additional calls to settings_errors() can be used to show errors even when the settings page is first accessed.