show error message for invalid tweet archive

routes.py

@@ -6,6 +6,7 @@ from lib import require_auth
 from model import Account, Session, Post, TwitterArchive
 from app import app, db, sentry
 import tasks
+from zipfile import BadZipFile
 
 @app.before_request
 def load_viewer():
@@ -68,9 +69,14 @@ def upload_tweet_archive():
     db.session.add(ta)
     db.session.commit()
 
+    try:
         tasks.chunk_twitter_archive(ta.id)
 
+        assert ta.chunks > 0
+
         return redirect(url_for('index', _anchor='recent_archives'))
+    except (BadZipFile, AssertionError):
+        return redirect(url_for('index', error='The file you uploaded is not a valid tweet archive. No posts have been imported.'))
 
 @app.route('/settings', methods=('POST',))
 @require_auth

templates/logged_in.html

@@ -2,6 +2,12 @@
 {% extends 'lib/layout.html' %}
 {% block body -%}
 
+{% if error %}
+<section class=banners>
+    <div class="banner error">{{ error|safe }}</div>
+</section>
+{% endif %}
+
 <section class=viewer>
 <p>Hello,
     <img class=avatar src="{{g.viewer.account.avatar_url}}"/>