Type error in file upload error handling

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Sentry link: https://new-sentry.gitlab.net/organizations/gitlab/issues/1786784/?alert_rule_id=53&alert_type=issue&notification_uuid=d8eaef3e-099c-46f7-b2de-3d849d9e2bd3&project=4&referrer=slack

Appears to be caused by lack of optional chaining here: https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/assets/javascripts/dropzone_input.js#L265