Improve error message in `WP_Block_Templates_Registry::unregister()` to include template name

[mukesh27]

Currently, when attempting to unregister a non-existent block template, WP_Block_Templates_Registry::unregister() returns a generic error message without reference to the template name. It use __( 'Template "%s" is not registered.' ) in error message without using template name.

Trac ticket: https://core.trac.wordpress.org/ticket/64072

• Updated the unregister method in WP_Block_Templates_Registry to ensure that the error message returned when trying to unregister a non-registered template includes the template name, making debugging easier.
• Added a new PHPUnit test to verify that unregister_block_template() returns a WP_Error containing the template name when attempting to unregister a template that is not registered. This test also checks for the correct usage notice.

[SergeyBiryukov]

In 61090:

Editor: Correct error message in WP_Block_Templates_Registry::unregister().

When attempting to unregister a non-existent block template, the error message will now include the template name.

Follow-up to [59073].

Props mukesh27, shailu25.
Fixes #64072.