Keep server details out of API answers

An unexpected error now sends a general message, and the log keeps the
text. A job no longer shows the path of its uploaded file. A cover is
cached only by the browser of the user.

tests/test_api.py

@@ -222,6 +222,7 @@ def test_the_cover_of_a_story(client):
     response = client.get(f"/api/stories/{story_id}/cover?v={version}")
     assert (response.status_code, response.mimetype, response.data) == (200, "image/png", PNG)
     assert response.cache_control.max_age == 365 * 24 * 60 * 60
+    assert response.cache_control.private and not response.cache_control.public
 
     # A new cover gets a new version, so the browser loads it.
     store.put_image(story_id, "https://example.com/new.jpg", "jpg", "image/jpeg", b"new", is_cover=True)