custom error message for workflow error

Hi,

Scenario:
There are 3 fields in my object- first name, last name, full name. The full name suhould be lastname+first name and need to be unique and auto populated. If the uniqueness fails it should show the error message with out saving the record, The error message should be "full name already exist please check".

When ever the record is getting created, the full name is created by workflow rule if there is no same fullname already, the record will get created, but if the full name already exists it will give error and it is a standard error message(
Error: Invalid Data.
Review all error messages below to correct your data.
Duplicate value on record: tstt (Related field: full name)
how can I keep the custom error meassage I need??

Thanks in advance