Testing PostgreSQL function

I'm moving an existing Sybase system to PostgreSQL and am having trouble with testing my functions. For example I have the following code

drop function if exists contract_line_ins(bigint, bigint, char(10), date, date, numeric(18, 0), numeric(18, 0), integer, bigint); 

create function contract_line_ins (
  in    _contract_id       bigint,
  in    _product_id        bigint,
  in    _number            char(10),
  in    _start_date        date,
  in    _end_date          date,
  in    _history_access    numeric(18, 0),
  in    _subscription_type numeric(18, 0),
  inout _rows_updated      integer,
  inout _id                bigint
)
as $$
declare
  _locus int = 1;    -- Set this in code to indicate current executing statement in exception
  _at text;          -- Used in exception handling

begin
  insert into contract_line(contract_id, product_id, number, start_date, end_date, history_access, subscription_type)
  values (_contract_id, _product_id, _number, _start_date, _end_date, _history_access, _subscription_type)
  returning _id;

  get diagnostics _rows_updated = row_count;

-- Exception handling
  exception when others then
    get stacked diagnostics _at = PG_EXCEPTION_CONTEXT;
    raise notice E'EXCEPTION\nError:   %\nMessage: %\nLocus:   %\nAt:      %', SQLSTATE, SQLERRM, _locus, _at;
end;
$$ language plpgsql;

I appreciate that having both return values is superflous in this case but I have been asked not to change parameters unless absolutely necessary.

I wrote the following test code

do language plpgsql $$
declare
  contract_id       bigint = 1;
  product_id        bigint = 2;
  number            char(10) = 'CONTRACT';
  start_date        date = '20160101';
  end_date          date = '20161231';
  history_access    numeric(18, 0) = 3;
  subscription_type numeric(18, 0) = 4;
  rows_updated      int;
  id                bigint;
begin
  perform contract_line_ins(contract_id, product_id, number, start_date, end_date, history_access, subscription_type, rows_updated, id);

  raise notice E'row count % id %', rows_updated, id;
end
$$

When I execute this test I get the following:

[2016-06-18 05:55:17] EXCEPTION
Error:   42601
Message: query has no destination for result data
Locus:   1
At:      PL/pgSQL function contract_line_ins(bigint,bigint,character,date,date,numeric,numeric,integer,bigint) line 7 at SQL statement
SQL statement "SELECT contract_line_ins(contract_id, product_id, number, start_date, end_date, history_access, subscription_type, rows_updated, id)"
PL/pgSQL function inline_code_block line 13 at PERFORM
[2016-06-18 05:55:17] row count <NULL> id <NULL>
[2016-06-18 05:55:17] completed in 43ms

What I don't understand is:

1. Why the "query has no destination for result data" message? I thought PERFORM was meant to prevent that
2. The exception is happening in my test code but is being raised in the function. I would have thought the function would only report exception that happened in its own body. Why is this happening?

Although I have found the PostgreSQL documentation excellent I just can't find out how to do this properly.

Any suggestions gratefully received.