Add class that encapsulates error in new Ci config

lib/gitlab/ci/config.rb

lib/gitlab/ci/config/node/configurable.rb

lib/gitlab/ci/config/node/entry.rb

lib/gitlab/ci/config/node/error.rb

lib/gitlab/ci/config/node/factory.rb

lib/gitlab/ci/config/node/script.rb

spec/lib/ci/gitlab_ci_yaml_processor_spec.rb

spec/lib/gitlab/ci/config/node/error_spec.rb

spec/lib/gitlab/ci/config/node/factory_spec.rb

spec/lib/gitlab/ci/config/node/global_spec.rb

spec/lib/gitlab/ci/config/node/script_spec.rb

spec/lib/gitlab/ci/config_spec.rb