1.) If you do this in a BTEQ script you could simple reduce the severity of the table does not exist error (don't have the number in my head) and drop it to a warning. Once you are done with your drops, restore the previous severity.
That way it does not matter if the table exists or not.
2.) If you do this for many tables you can also generate the SQL for the drops out of the dbc tables. If a table does not exist, it will simply not get included in the generate SQL as the dbc tables would not contain records for this table.