Nav Menu throws a fatal error for invalid taxonomies

[ptahdunbar]

Reported by Ron from ​wp-testers mailinglist:

When I go to Menus I get the following error:
" Object of class WP_Error could not be converted to string in
\...deleted...\wp-includes\formatting.php
on line 433"

Patch fixes this issue by checking if the terms retrieved by get_terms() is a WP_Error object.

[nacin]

(In [14048]) Account for WP_Error from get_terms() in the nav menu tax metabox. fixes #12949, props ptahdunbar.

[nacin]

get_term_link() can also return WP_Error. We're then running esc_attr on that, and it's causing the same issue.

[ptahdunbar]

nav_menu_term_url_bug.dif returns a blank url if get_term_link() throws a wp_error.

[ryan]

(In [14599]) Check for WP_Error. Props ptahdunbar. fixes #12949