term_description() shouldn't return error objects

[filosofo]

term_description() and its wrappers like category_description() are meant to be used in templates, so they shouldn't return WP_Error objects.

However, if you call term_description() for a non-existent term you get a fatal error as PHP attempts to treat the WP_Error object as a string.

Patch instead returns an empty string when an error bubbles up to term_description().

[scribu]

Looks good.

[ryan]

[13813]