🐞 Bug Report: Fatal error when an existing upgrader_source_selection returns an error

[johnbillion]

Detailed Description

The upgrader_source_selection filter can accept and return a string or a WP_Error, but the methods in FAIR Connect that hook into this filter use a string type declaration. If another plugin is using the upgrader_source_selection filter on a lower priority and returns a WP_Error then FAIR Connect triggers a fatal type error.

Steps to reproduce

add_filter( 'upgrader_source_selection', fn() => new WP_Error( 'oh', 'no' ), 9 );

Put this code into a place on a site and then attempt to perform an update on a plugin.