How to execute PostgreSQL RAISE command dynamicall

How to raise error from PostgreSQL SQL statement if some condition is met?
I tried code below but got error.

CREATE OR REPLACE FUNCTION "exec"(text)
  RETURNS text AS
$BODY$ 
    BEGIN 
      EXECUTE $1; 
      RETURN $1; 
    END; 
$BODY$
  LANGUAGE plpgsql VOLATILE;

-- ERROR:  syntax error at or near "raise"
-- LINE 1: raise 'test' 

SELECT exec('raise ''test'' ') WHERE TRUE

In real application TRUE is replaced by some condition.

Update

I tried to extend answer to pass exception message parameters. Tried code below but got syntax error. How to pass message parameters ?

CREATE OR REPLACE FUNCTION exec(text, variadic ) 
  RETURNS void LANGUAGE plpgsql AS 
$BODY$  
BEGIN  
   RAISE EXCEPTION  $1, $2;  
END;  
$BODY$; 

SELECT exec('Exception Param1=% Param2=%', 'param1', 2 );

You cannot call RAISE dynamically (with EXECUTE) in PL/pgSQL - that only works for SQL statements, and RAISE is a PL/pgSQL command.

Use this simple function instead:

CREATE OR REPLACE FUNCTION f_exec(text)
  RETURNS void LANGUAGE plpgsql AS
$BODY$ 
BEGIN 
   RAISE EXCEPTION '%', $1; 
END; 
$BODY$;

Call:

SELECT f_exec('My message is empty!');

I wrote more in this related answer on dba.SE: explanation, links, more options for the function, context (including line number) from the function call etc.

Additional answer to comment

CREATE OR REPLACE FUNCTION f_exec1(VARIADIC text[]) 
  RETURNS void LANGUAGE plpgsql AS 
$BODY$  
BEGIN  
   RAISE EXCEPTION 'Reading % % %!', $1[1], $1[2], $1[3];
END;  
$BODY$;

Call:

SELECT f_exec1('the','manual','educates');

VARIADIC is not a data type, but an argument mode.
Elements have to be handled like any other array element.
To use multiple variables in a RAISE statement, put multiple % into the message text.

The above example will fail if no $3 is passed to the call. You'd have to assemble a string from the variable number of input elements. Example:

CREATE OR REPLACE FUNCTION f_exec2(VARIADIC _arr text[]) 
  RETURNS void LANGUAGE plpgsql AS 
$BODY$  
DECLARE
   _msg text := array_to_string(_arr, ' and '); -- simple string construction
BEGIN  
   RAISE EXCEPTION 'Reading %!', _msg;
END;  
$BODY$;

Call:

SELECT f_exec2('the','manual','educates');

I doubt you need a VARIADIC parameter for this at all. Read the manual here.
Instead, define all parameters, maybe add defaults:

CREATE OR REPLACE FUNCTION f_exec3(_param1 text = ''
                                  ,_param2 text = ''
                                  ,_param3 text = 'educates') 
  RETURNS void LANGUAGE plpgsql AS 
$BODY$  
BEGIN  
   RAISE EXCEPTION 'Reading % % %!', $1, $2, $3;
END;  
$BODY$;

Call:

SELECT f_exec3('the','manual','educates');
SELECT f_exec3();  -- defaults kick in