`media_sideload_image()` tries to unlink a WP_Error object.

[georgestephanis]

Traced back to: https://core.trac.wordpress.org/changeset/8959/trunk/wp-admin/includes/media.php by @noel

Long story short, the conditional block will only be entered if $tmp is a WP_Error object, then once in, it will try to unlink $file_array['tmp_name'] -- which was just set equal to $tmp and is therefore also a WP_Error object and not a file path, so unlink will always fail!

Also, instead of setting $file_array['tmp_name'] to an empty string and causing it to fail and pass back a WP_Error later, let's just pass back the WP_Error directly.

[georgestephanis]

Related: #7732

[wonderboymusic]

In 28567:

In media_sideload_image(), check the returned value of download_url() earlier to avoid attempting to unlink() an instance of WP_Error.

Props georgestephanis.
Fixes #28204.