Cancel a download job when deleted from web UI

src/plugin/downloader.cr

@@ -66,6 +66,8 @@ class Plugin
       fail_count = 0
 
       while page = plugin.next_page
+        break unless @queue.exists? job
+
         fn = process_filename page["filename"].as_s
         url = page["url"].as_s
         headers = HTTP::Headers.new
@@ -109,6 +111,12 @@ class Plugin
         end
       end
 
+      unless @queue.exists? job
+        Logger.debug "Download cancelled"
+        @downloading = false
+        return
+      end
+
       Logger.debug "Download completed. #{fail_count}/#{pages} failed"
       writer.close
       filename = File.join File.dirname(zip_path), File.basename(zip_path,