tgtadm: clarify tid==0 error message

The problem with supporting tid 0 is unclear, but this at least
clarifies the message with "you can't use 0" as opposed to "you have
to supply the tid option", since in the case of 0, the user *has*
supplied the option, but with an unacceptable value.

Signed-off-by: Dan Mick <dan.mick@inktank.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>