Fix three bugs found reviewing this session's changes

- notes.js's ICON_LABELS wasn't extended for the shop_supermarket/
  shop_convenience keys icons.js added — the note icon picker showed
  the raw internal key as the title instead of a real label.
- Plugins.run mislabeled a malformed plugin.json as "plugin did not
  print valid JSON" — a message written for bad script stdout, not a
  bad manifest, since both paths shared one rescue.
- overpass_pois.rb's description_for joined raw OSM tag values with
  "\n" — an operator/opening_hours tag with an embedded newline could
  inject an extra line into the popup that gpx.js's formatDescription
  (built for p4n2gpx's own well-formed lines) would render as its own
  independent, legitimate-looking link. Tag values are stripped of
  embedded newlines before joining.

public/js/notes.js

@@ -24,6 +24,8 @@ const ICON_LABELS = {
   leisure_fishing: "Fishing",
   sport_climbing: "Climbing",
   route_hiking: "Hiking",
+  shop_supermarket: "Supermarket",
+  shop_convenience: "Convenience store",
 };
 
 const NEW_LAYER_VALUE = "__new__";