Fixes a potential PHP error in the Clef logout hook handling.

If the logout hook handler fails to reach the Clef servers with the URL request, it will return a WP_Error. Previously, we didn't handle the error and that caused a PHP server error. With this commit, we handle it correctly.
Merge pull request #178 from clef/fix-logout-hook-error-handling

Fixes a PHP error in the Clef logout hook handling