handle file error using an exception

prolefeeder.py

@@ -34,12 +34,12 @@ def download():
         form.start_time.data = datetime.now().replace(minute=0, second=0) - timedelta(hours=1)
         form.length.data = 60
     elif form.validate_on_submit():
-        output_filename = prepare_download(form)
-        if output_filename:
+        try:
+            output_filename = prepare_download(form)
             flash('The download should start immediately.', 'success')
             return render_template('download.html', form=form, filename=output_filename)
-        else:
-            flash('Error preparing download!', 'warning')
+        except ValueError as e:
+            flash('Error preparing download: {}'.format(e), 'warning')
     else:
         flash('Error in form!', 'warning')
     return render_template('download.html', form=form)
@@ -79,7 +79,7 @@ def prepare_download(form):
         if source['start_time'] <= form.start_time.data < source['start_time'] + source['length']:
             start_index = i
     if start_index is None:
-        return None
+        raise ValueError('Could not find start file')
     sources = sources[start_index:]
 
     # Seek into the first file