Can trigger procedures return value

Hi all,
Can trigger procedures can return
values.

Actually I need a trigger that should
not allow rows to be updated in a particular
table on a specific condition.(like users
other than 'manager' should not be allowed to
update that table')

If the user other than 'manager' tries to update
that table, I need a error number that should be
returned by trigger procedure to prompt the
user that he/she is not a valid user.

If it is not possible to return error from
trigger procedure, what could be the possible
way to solve the above problem.

regards,
Deepa. K