SQL> ed
Wrote file afiedt.buf

  1  create or replace procedure check_user (uname in varchar2)
  2  as
  3  a dba_users.username%type;
  4  begin
  5  select username into a from dba_users where username=upper(uname);
  6  dbms_output.put_line('user '||a||' already exist');
  7  exception
  8     when no_data_found then
  9     dbms_output.put_line('NO SUCH USER');   
 10* end;
SQL> /

Procedure created.

SQL>  exec check_user('MAG');
user MAG already exist

PL/SQL procedure successfully completed.

SQL>  exec check_user('Gandolf');
NO SUCH USER

PL/SQL procedure successfully completed.