Trigger to track user password changes in table

I need to create a trigger which tracks user password changes into a table my_table. I have a database user who has alter user privileges and would like him to change users who belong to his group. That is, when the user changes the password for a user, the trigger fires and inserts info into my_table. In case this user tries to change another user's password, the trigger should fire again telling him 'cannot alter the user.'

I'm enclosing the trigger code I wrote:

 create or replace trigger LOGG_TRAIL
  before alter on database
  when (ora_dict_obj_type = 'USER' and 
ora_dict_obj_name != user
        and sys_context('USERENV','ISDBA') = 'FALSE' ); 
declare
  by_user  varchar2(30);
  to_user  varchar2(30);
  vprofile varchar2(30);
begin
  select to.PROFILE into vprofile
  from DBA_USERS by, DBA_USERS to
  where by.USERNAME = user
  and to.USER_NAME = ora_dict_obj_name
  and by.PROFILE = to.PROFILE;
 
  insert into TRAIL_TEMP (USER_ALTERED, PASSWORD_MODIFIED_BY, 
PASSWORD_MODIFIED_DATE, PROFILE)
  values (ora_dict_obj_name, user, sysdate, vprofile); exception
  when others then
    raise_application_error (-20001,'can''t alter user'); end; /

This gives errors. Can you please check it? Your help will be highly appreciated.