Add test to check error case

Author: JulianBustamante

features/post.feature

@@ -267,6 +267,12 @@ Feature: Manage WordPress posts
       {POST_ID}
       """
 
+    When I run `wp post url-to-id non-existent-url`
+    Then STDOUT should contain:
+      """
+      Could not get post with url 'non-existent-url'.
+      """
+
   Scenario: Update a post from file or STDIN
     Given a content.html file:
       """

src/Post_Command.php

@@ -897,7 +897,7 @@ public function url_to_id( $args, $assoc_args ) {
 		$post_id = url_to_postid( $args[0] );
 
 		if ( ! $value ) {
-			WP_CLI::error( "Could not get post with url '{$args[0]}'." );
+			WP_CLI::error( "Could not get post with url '$args[0]'." );
 		}
 
 		WP_CLI::print_value( $post_id, $assoc_args );