title: How to create a migration to remove an index only if it exists, rather than throwing an exception if it doesn't?
body: <p>Right now, the current migration might fail, if the <code>books</code> table doesn't have <code>created_at</code> or <code>updated_at</code> fields:</p>
  
  <pre><code>class AddTimestampIndexes &lt; ActiveRecord::Migration
    def up
      remove_index :books, :created_at
      remove_index :books, :updated_at
  
      add_index  :books, :created_at
      add_index  :books, :updated_at
    end
  
    def down
      remove_index :books, :created_at
      remove_index :books, :updated_at
    end
  end
  </code></pre>
  
  <p>Does <code>remove_index</code> take any options to silently proceed if it fails to remove the index rather than raising an error?</p>