@Override
    public AuthenticatedUser updateAuthenticatedUser(AuthenticatedUser authenticatedUser,
            Credentials credentials) throws GuacamoleException {

        // Ignore auth attempts if no auth provider could be loaded
        if (authProvider == null) {
            logger.warn("Reauthentication attempt ignored because the relevant "
                    + "authentication provider could not be loaded. Please "
                    + "check for errors earlier in the logs.");
            return null;
        }

        // Delegate to underlying auth provider
        return authProvider.updateAuthenticatedUser(authenticatedUser, credentials);

    }
Logs a warning that the authentication process will be entirely aborted
due to an internal error, advising the administrator to set the
"skip-if-unavailable" property if error encountered is expected and
should be tolerated.
guacamole/src/main/java/org/apache/guacamole/extension/AuthenticationProviderFacade.java