Issue 625

[tiago-loureiro]

Return proper error response in case a team has reached its size limit and a user tries to join. We check this in 2 places because:

• We want to fail as early as possible and avoid adding users to the user table that will not be able to verify their account.
• There is always a potential race condition when 2 users register so we must assume that addTeamMember can potentially fail.