WP-r43578: Customize: Safeguard a check on the customize_validate_{$setting_id} filter value to ensure it is a WP_Error.

[PaoloFalomo]

WP-r43578: Customize: Safeguard a check on the customize_validate_{$setting_id} filter value to ensure it is a WP_Error.

While the filter is documented to only support a WP_Error, it has been a common practice to return true in a validation function if no errors have occurred. This was already caught when the same filter was executed in WP_Customize_Setting, it was however missing in WP_Customize_Manager::validate_setting_values().

Fixes https://core.trac.wordpress.org/ticket/44809.

Conflicts:
src/wp-includes/class-wp-customize-manager.php

Merges https://core.trac.wordpress.org/changeset/43578 / WordPress/wordpress-develop@1e4e60f11c to ClassicPress.

[nylen]

There were 16 errors:

1) Tests_WP_Customize_Manager::test_changeset_data
Error: Call to undefined method WP_Error::has_errors()

etc.

Maybe this changeset depends on another one that has not been merged yet?

[Mte90]

Looking the has_errors is a method added in 5.1 https://github.com/WordPress/WordPress/blob/master/wp-includes/class-wp-error.php#L171
So probably is better to use https://core.trac.wordpress.org/changeset/44392 that is for 5.0.
This can be closed and we need a backport of that changeset that should not create issues.
We need to be more careful on selecting the changeset but in this case probably the issue is the release cycle of 5.0/5.1 where they moved a lot of changeset so the last one in the ticket is not the right rule.

[Mte90]

CLosed for #398