Bugfix for %%ct_desc_<custom-tax-name>%%

Fixed bug in processing of %%ct_<custom-tax-name>%% as [reported by Joy](http://wordpress.org/support/topic/plugin-dies-when-processing-ct_desc_) - props [Jrf](http://profiles.wordpress.org/jrf).

N.B.1: Added a text string, so the .pot file will have to be updated.
N.B.2: Even though the replace code isn't being run on the WPSEO admin page (or is it ?), the error notice should still display when people save a post of the custom post type for which the invalid description has been set. Am I correct in assuming that ?