Safeguard `has_errors()` check in extra Customizer validation

[flixos90]

The customize_validate_{$setting_id} filter is run in both the WP_Customize_Setting class, but then again in WP_Customize_Manager, to account for cases where someone might have overridden that method without maintaining the filter.

I ran into an issue earlier causing a fatal error: While the documentation for customize_validate_{$setting->id} states that the parameter is a WP_Error, several plugins and themes have been returning true in case of success. This is correctly taken care of in WP_Customize_Setting, but not when the filter is executed again in WP_Customize_Manager::validate_setting_values(). has_errors() is called on the filter result, which however (unfortunately) may not always be a WP_Error object. We should add a is_wp_error() check to be safe.

[flixos90]

44809.diff​ fixes the issue.

[westonruter]

Good change. Feel free to commit!

Introduced in #37638.

[flixos90]

In 43578:

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 #44809.

[flixos90]

This needs to be backported to 4.9.9.

[SergeyBiryukov]

In 43619:

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().

Props flixos90.
Merges [43578] to the 4.9 branch.
Fixes #44809.

[pento]

[43619] needs to be reverted from the 4.9 branch. This ticket can then be moved to the 5.0.1 milestone.

[SergeyBiryukov]

In 43702:

Customize: Revert [43619] from the 4.9 branch.

This change is out of the 4.9.x scope, and will be reintroduced in 5.0.x.

See #44809.

[desrosj]

In 44392:

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().

Props flixos90.

Merges [43578] to the 5.0 branch.
Fixes #44809.