Return error messages from Validate::Abilities

After Rails 6.1 changes related to ActiveModel errors
the `#add` method doesn't return errors anymore.

Let's just return the errors for backward compatibility

lib/gitlab/ci/pipeline/chain/helpers.rb

@@ -19,6 +19,8 @@ module Gitlab
             # polluted with other unrelated errors (e.g. state machine)
             # https://gitlab.com/gitlab-org/gitlab/-/issues/220823
             pipeline.errors.add(:base, message)
+
+            pipeline.errors.full_messages
           end
 
           def warning(message)