Log the failing SQL statement in vectorization task

Unique violations on packages_name_key happen on a statement that
cannot logically touch name - log the exact query and a longer
backtrace to identify what really fails.

lib/tasks/vectorize_packages.rake

@@ -30,6 +30,13 @@ namespace :debshots do
     done = 0
     failed = 0
 
+    # Remember the most recent SQL statement so failures can be traced
+    # to the exact query that triggered them.
+    last_sql = nil
+    ActiveSupport::Notifications.subscribe('sql.active_record') do |*args|
+      last_sql = args.last[:sql]
+    end
+
     scope.find_each do |package|
       begin
         raise ArgumentError, 'no description text' if package.embedding_text.blank?
@@ -41,7 +48,8 @@ namespace :debshots do
         # enough detail to diagnose it and move on.
         failed += 1
         logger.error "Failed for package #{package.name}: #{e.class}: #{e.message}"
-        logger.error e.backtrace.first(3).join("\n") if e.backtrace
+        logger.error "Last SQL: #{last_sql}"
+        logger.error e.backtrace.first(15).join("\n") if e.backtrace
       end
 
       processed = done + failed