Disjointed WP_Error documentation and handling in wp-signup.php

[jeremyfelt]

Most of the documentation for errors throughout wp-signup.php has the $errors data typed as array rather than WP_Error.

In show_user_form() and show_blog_form(), an empty string is set as default for the $errors param, but it is used as a WP_Error object throughout.

The likelihood of a fatal Call to a member function get_error_message() on string here is low, because this code is really self contained and somebody would need to filter responses to remove existing WP_Error objects completely. That said, we can still clean it up.

[jeremyfelt]

In 37547:

Multisite: Clean up WP_Error documentation and handling in wp-signup.php

Fixes #36927.