Say ID of item that does not exist for migrations

Putting the item id in your face may help you realise you made a typo or
similar mistakes.

src/item_factory.cpp

@@ -1365,7 +1365,8 @@ void Item_factory::finalize_item_blacklist()
         const migration *parent = nullptr;
         for( const migration &migrant : migrate.second ) {
             if( m_templates.count( migrant.replace ) == 0 ) {
-                debugmsg( "Replacement item for migration %s does not exist", migrate.first.c_str() );
+                debugmsg( "Replacement item (%s) for migration %s does not exist", migrant.replace.str(),
+                          migrate.first.c_str() );
                 continue;
             }
             // The rest of this only applies to blanket migrations