Trapping 'invalid input syntax for integer'

Is there a way to trap this error in plpgsql code?

I have a function that accepts integer and character varying. Inside
that function I need to cast that varchar to integer. Of course,
sometimes that is not possible.
When I run function like that, I get this errror:

fidel=# select * from get_account_info_by_tan(1, 'mario');
ERROR:  invalid input syntax for integer: "mario"
CONTEXT:  SQL statement "SELECT  user_id FROM user_tans WHERE user_tan =$1 ::bigint"
PL/pgSQL function "get_account_info_by_tan" line 8 at assignment

Now, I know I could change the SELECT so it looks like:

SELECT user_id FROM user_tans WHERE user_tan::varchar = $1

But, is there a way to trap above error usin EXCEPTION WHEN keyword in
plpgsql?
Mike