User Badges plugin should check for WP_Error in ::get_author_badge_info()

[SergeyBiryukov]

​wp_get_post_terms() can return an empty array if no terms were found, or a WP_Error object if the taxonomy does not exist.

User_Badges\Plugin::get_author_badge_info() has a check for the former, but not for the latter. Normally, this code only runs when the taxonomy exists, but it can still lead to a fatal error if WPORG_SUPPORT_FORUMS_BLOGID is not defined (e.g. to differentiate between English and Rosetta forums on a local install):

Fatal error: Uncaught Error: Cannot use object of type WP_Error as array in wp-content/plugins/wporg-bbp-user-badges/inc/class-plugin.php:142

[coffee2code]

In 5156:

Support Forums, User Badges: Also check that wp_get_post_terms() did not return WP_Error for taxonomy not existing.

Props SergeyBiryukov.
Fixes #2591.