When revision body missing on disk, should be able to redownload it without restarting UI

[davidfstr]

The following test characterizes the current (unusual) behavior:

async def test_given_rr_is_downloaded_but_revision_body_missing_when_expand_rrn_then_shows_error_node_and_redownloads_rr() -> None:
    ...
    
    # Expand RootResourceNode and ensure it has an _ErrorNode child
    # 
    # TODO: In the future, block on the redownload finishing
    #       and list the links in the redownloaded revision,
    #       WITHOUT needing to reopen the project later
    ...