Only silence REST errors if the REST server is added (#10903)

* Only silence REST errors if the REST server is added

If the REST server is not available in a code run, Gutenberg will fatal with `Fatal error: Uncaught Error: Call to undefined function gutenberg_silence_rest_errors()`

`gutenberg_silence_rest_errors` is defined in `lib/rest-api.php`, which is only loaded if `class_exists( 'WP_REST_Controller' )` in `load.php`.

This PR adds the same `class_exists()` test to the call to `gutenberg_silence_rest_errors()` in `gutenberg.php`.

gutenberg.php

@@ -164,7 +164,9 @@ function gutenberg_pre_init() {
 
 	require_once dirname( __FILE__ ) . '/lib/load.php';
 
-	gutenberg_silence_rest_errors();
+	if ( function_exists( 'gutenberg_silence_rest_errors' ) ) {
+		gutenberg_silence_rest_errors();
+	}
 
 	add_filter( 'replace_editor', 'gutenberg_init', 10, 2 );
 }