make checking for illegal filenames explicit

prolefeeder.py

@@ -55,12 +55,9 @@ def download():
 def download_file(filename):
     """Download an output file"""
 
-    # Get attachment filename. This also makes sure that the user only downloads
-    # (and removes) a file generated by us.
-    try:
-        attachment_filename = attachment_filenames.pop(filename)
-    except KeyError:
+    if filename not in attachment_filenames:
         abort(404)
+    attachment_filename = attachment_filenames.pop(filename)
 
     fh = open(os.path.join(app.config['TMP_DIR'], filename), 'rb')
     os.remove(os.path.join(app.config['TMP_DIR'], filename))