OK, without a doubt I have determined that the lines causing me problems are:

EXEC sp_addrolemember N'db_datareader', N'[DomainUsersAccess]'

EXEC sp_addrolemember N'db_datawriter', N'[DomainUsersAccess]'

Those 2 lines generate the error messages:

I just tried running those 2 lines of code again, and it failed with the same message. I don't understand why, because I'm running them connected to the database (ASI, in this case) where the DomainUsersAccess role exists. And I think I now know where that "Line 75" in the error message is coming from. I believe it's from the sp_AddRoleMember stored procedure; it's not from my 2 lines of SQL script.