title: Handle Angular Proxy 404 [intended] error
body: <p>I have a simple proxy configuration as follows:</p>
  <pre class="lang-json prettyprint-override"><code>{
    &quot;/preview&quot;: {
      &quot;target&quot;: &quot;https://localhost:44323&quot;,
      &quot;secure&quot;: false,
      &quot;pathRewrite&quot;: {
        &quot;^/preview&quot;: &quot;&quot;
      },
      &quot;changeOrigin&quot;: true
    }
  }
  </code></pre>
  <p>But the following URI cannot be reached (this is intended). I want to implement some specific behaviour on this. But I can't catch it with error interceptor &amp; didn't find any examples on the internet.</p>
  <p>I want to insert some site (that can be offline) as an iFrame in site. If site is offline want to display some custom page.</p>
  <p>What I get instead of content:</p>
  <pre><code>Error occured while trying to proxy to: localhost:44323/?eventId=e4813a1c-918f-4e74-b525-7d5817256ac1
  </code></pre>
  <p>P.S. Potential use of this is much broader, than is provided.</p>