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

[mukeshpanchal27]

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.

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