add error page for when something goes wrong

src/ErrorPage.svelte

@@ -0,0 +1,30 @@
+<svelte:head>
+  <title>Uh-oh!</title>
+</svelte:head>
+
+<div class="error-page">
+  <h1>Uh-oh!</h1>
+  <p>
+    It looks like it's not possible to retrieve the contents of the API documentation
+    at the moment. If you're the owner of this site, make sure that your
+    <a href={window.INSOMNIA_URL} target="_blank">Insomnia JSON file</a> is accessible.
+  </p>
+  <p>
+    The developer console of your browser might have more things to say about this error.
+  </p>
+</div>
+
+<style>
+  .error-page {
+    width: 760px;
+    margin: 0 auto;
+    margin-top: 60px;
+  }
+
+  @media only screen and ( max-width: 760px ) {
+    .error-page {
+      width: auto;
+      padding: 15px;
+    }
+  }
+</style>

src/main.js

@@ -1,4 +1,5 @@
 import App from './App.svelte';
+import ErrorPage from './ErrorPage.svelte';
 import CuteConfig from './lib/cuteConfig';
 
 async function app() {
@@ -11,24 +12,32 @@ async function app() {
 
   window.INSOMNIA_URL = url;
 
-  // eslint-disable-next-line no-undef
-  const json = await fetch(url, {
-    method: 'GET',
-    credentials: 'same-origin',
-    headers: {
-      Accept: 'application/json',
-      'Content-Type': 'application/json'
-    }
-  }).then(res => res.json());
-
-  const insomniaConfig = new CuteConfig(json).generate();
-
-  return new App({
-    target: root,
-    props: {
-      config: insomniaConfig
-    }
-  });
+  try {
+    // eslint-disable-next-line no-undef
+    const json = await fetch(url, {
+      method: 'GET',
+      credentials: 'same-origin',
+      headers: {
+        Accept: 'application/json',
+        'Content-Type': 'application/json'
+      }
+    }).then(res => res.json());
+
+    const insomniaConfig = new CuteConfig(json).generate();
+
+    return new App({
+      target: root,
+      props: {
+        config: insomniaConfig
+      }
+    });
+  } catch (err) {
+    console.error(err);
+
+    return new ErrorPage({
+      target: root
+    });
+  }
 }
 
 export default app();