new feature test for calling cache pluck command with invalid key

Author: petitphp

features/cache-pluck.feature

@@ -31,3 +31,10 @@ Feature: Pluck command available for the object cache
       """
       bar_custom
       """
+
+    When I try `wp cache pluck unknown_key test`
+    Then STDERR should be:
+      """
+      Warning: No object found for the key "unknown_key" in group "default"
+      """
+

src/Cache_Command.php

@@ -444,7 +444,8 @@ public function pluck( $args, $assoc_args ) {
 		$value = wp_cache_get( $key, $group );
 
 		if ( false === $value ) {
-			WP_CLI::halt( 1 );
+			WP_CLI::warning( 'No object found for the key "' . $key . '" in group "' . $group . '"' );
+			exit;
 		}
 
 		$key_path = array_map(