500 error when trying to sign up a roster with a player on another signed up roster

[coreobs]

As the issue title says, when trying to sign up a roster with a player or players (in a team of ants in their prime's teams' case, they have multiple on other rosters) that are already signed up on other rosters, it just errors rather than giving a graceful "You cannot sign up x player" or something to that effect.

[Shigbeard]

Further development: As woozy learned today, we get the same 500 error when attempting to sing up a roster with a player or players who have active league bans.

This could indicate that there's a general failure when attempting to display an error for signing up with players who are otherwise ineligible to sign up, not just players in the same league but a different roster.