Members: when validating user signup, if `wp_insert_user` returns a `WP_Error` object, use it.

Prior to this change, we were replacing WP's helpful error message with a rather generic and unhelpful BP error message.
Fixes #6007, props Mamaduka

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9200 cdf35c40-ae34-48e0-9cc9-0c9da1808c22