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_).

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 ?

##### Update:

Just did a quick check - this solves a number of other forum threads as well, amongst which:
* http://wordpress.org/support/topic/wp_error-could-not-be-converted-to-string-in-wordpress-seo
* http://wordpress.org/support/topic/plugin-wordpress-seo-by-yoast-wp_error-using-ct_desc
* http://wordpress.org/support/topic/taxonomy-breaks-the-plugin

Actually, the second link suggests an alternative solution, will have a look & compare & update this pull request if relevant.

##### Update2:

Double check was useful. Really fixed now ;-)