Allow wpmu_create_user to return a WP_Error

[johnbillion]

The wpmu_create_user() function calls wp_create_user() which returns a descriptive WP_Error object upon failure, but this error isn't passed on as the return value from wpmu_create_user() which instead returns boolean false.

A $wp_error parameter should be added to wpmu_create_user() (similar to wp_insert_post()) in order to instruct the function to return the WP_Error object when there's a failure.

This will allow, for example, WP-CLI to show a more descriptive error message when there is a failure when attempting to create a user on Multisite. Currently it has to show a generic Unknown error creating new user message.

#21910 is related, but hasn't seen any movement in years, so for now let's add this parameter.

Return WP_Error object on failure from wpmu_create_user()

Trac ticket: https://core.trac.wordpress.org/ticket/50880

[johnbillion]

As per ​discussion on Slack, let's forget about changing this.