[Error Budget] Investigate `MaxAdditionalSeatsExceededError`

From: #4918 (comment 1124235188)
Sentry: https://sentry.gitlab.net/gitlab/customersgitlabcom/?query=is%3Aunresolved+lastSeen%3A%2B1d+maxadditional

This error seems like valid business logic and not an application exception. We should stop raising an error where appropriate so we're not reporting these as errors.