title: Finding errors from Rails collection<< association assignment
body: <p>In the event that <code>collection&lt;&lt;</code> for a <code>has_many</code> association on Rails fails, how to find the errors?</p>
  <p>For example:</p>
  <pre class="lang-rb prettyprint-override"><code>@author.books &lt;&lt; @book1
  </code></pre>
  <p>If this was to fail, how to catch the errors and return to user?</p>
  <p>Thanks</p>