fix tweet archive error banner placement

static/style.css

@@ -103,6 +103,10 @@ body > section > .banner {
     background: #eec;
 }
 
+form {
+    margin: 1em auto;
+}
+
 .banner form {
     display: inline;
 }

templates/logged_in.html

@@ -68,14 +68,15 @@
     <a href="https://twitter.com/settings/account">Twitter's settings page</a>,
     and upload it here.
     </p>
-    <form action='{{url_for('upload_tweet_archive')}}' method='post' enctype='multipart/form-data'>
-        <input type="file" name='file'><input type="submit" value="Upload">
-    </form>
 
     {% if tweet_archive_failed %}
     <div class="banner error">The file you uploaded is not a valid tweet archive. No posts have been imported.</div>
     {% endif %}
 
+    <form action='{{url_for('upload_tweet_archive')}}' method='post' enctype='multipart/form-data'>
+        <input type="file" name='file'><input type="submit" value="Upload">
+    </form>
+
 {% if g.viewer.account.twitter_archives %}
     <h3 id="recent_archives">Recent archives</h3>
     <ul>