typing: Add assertions for report returned from notify_server_error.
typing: Add assertions for report returned from notify_server_error.

We don't have a specific type for the reports returned from the error
logging handlers. The check is necessary as they are currently typed
as `Dict[str, object]` in `run_handler`.

Signed-off-by: Zixuan James Li <74923aee05c23ca90566381b5d6b0cd9451b6054@gmail.com>

rht/zulip,zulip/zulip,rht/zulip,andersk/zulip,rht/zulip,zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,rht/zulip,zulip/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,andersk/zulip