get_plugin_option() now returns empty string

Amended get_plugin_option() to return an empty string if the option
isn’t found (previously $options could theoretically be ‘false’ -
though highly unlikely)
Return WP_Error if _create_user() fails

In _create_user(), if wp_insert_user() fails return a WP_Error object
(this might happen if the email address is associated with an existing
user account)