Explicit failure for wrong settings importation formats.

src/bg/Settings.js

@@ -15,7 +15,8 @@ var Settings = {
         return await this.importSettings(json);
       }
     } catch (e) {
-      return await this.importLists(data);
+      if (data.includes("[UNTRUSTED]")) await this.importLists(data);
+      else throw e;
     }
   },
 

src/ui/options.js

@@ -57,6 +57,8 @@
           await UI.importSettings(fr.result);
         } catch (e) {
           error(e, "Importing settings %s", fr.result);
+          alert(e);
+          return;
         }
         location.reload();
       }