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_flows.py

@@ -145,8 +145,9 @@ def test_download_builds_from_stored_chapters_without_changing_them(client, stor
     response.close()
 
 
-def test_unexpected_backend_error_returns_json(client, monkeypatch):
+def test_unexpected_backend_error_returns_json(client, monkeypatch, caplog):
     monkeypatch.setattr(o, "list_stories", Mock(side_effect=RuntimeError("broken")))
     response = client.get("/api/stories")
     assert response.status_code == 500
-    assert response.json == {"error": "RuntimeError: broken"}
+    assert response.json == {"error": "Bindery hit an unexpected error. The log has the details."}
+    assert "RuntimeError: broken" in caplog.text