appdata: translate=no properties - #26

…s installed

install_plugin returns a meaningful result (True | 404 | NoConnectionError,
now also 400 for rejected ids) but almost no call site honored it:

- PluginPreview.install() discarded the result and unconditionally flipped
  the button to 'installed'; update() deregistered the old version before
  the fallible download, so a failure left the plugin unregistered until
  restart even with its files intact. install() now only advances the
  install state on success (marshalled via GLib.idle_add) and notifies on
  failure; a failed update re-registers the on-disk plugin via the new
  StoreBackend.reload_installed_plugins() recovery helper.
- update_all_plugins returned len(plugins_to_update) regardless of
  outcome; update_all_icons/-wallpapers ignored results entirely (their
  install_* helpers swallowed download_repo's return). All three now
  count only real successes, and a failed plugin update triggers the
  recovery reload once after the loop.
- update_everything's NoConnectionError check omitted n_wallpapers -- a
  wallpapers-only failure raised TypeError on the sum, and @log.catch in
  _update_all_assets then left the busy cursor stuck. All three legs are
  checked, and the toast now reports the real count (or the failure)
  instead of an unconditional "all assets updated".
- app.py treated 404 (truthy int) as success ("if not success"); success
  is now exactly True.
- MissingRow handled only 404, missing NoConnectionError; and its
  show_install_error removed the "error" css class instead of adding it,
  so failure styling never showed. Both fixed.
- Onboarding ignored results; failed installs are now logged and surfaced
  on the progress bar.
- download_repo's devel path dropped clone_repo's result (git-missing 404
  was lost); clone_repo now returns 200 on success and download_repo
  propagates it, giving download_repo a uniform 200|404|NoConnectionError
  contract.

Covered by tests/scenario_store_install_contract.py (network-free).

Closes StreamController#26