1.4.0-beta release - #20

A failed thumbnail fetch (429 storm, offline) made prepare_icon,
prepare_wallpaper and prepare_sd_plus_bar_wallpaper return the
NoConnectionError, which process_store_data's data_class filter then
dropped -- the whole pack silently vanished from the tab, and an all-fail
page showed 'Nothing here' instead of the connection-error page.
prepare_plugin already survived this by listing the entry without an
image; replicate that fallback at the other three thumbnail sites.
A failed manifest fetch still drops the entry (no id/name to list).

Closes StreamController#20