cbc_design(method = full/orthogonal) returns a design with NA

[shigono]

Hello,

For the full and orthogonal methods, cbc_design() returns missing experimental designs when the number of possible profiles is less than the number of trials.
Though it will only rarely occur, it could be helpful if an error raises instead.
(this request is based on #27 (comment))

Thank you ,
Shigeru