Explain RAISE_APPLICATION_ERROR

RAISE_APPLICATION_ERROR(num, message [, keep_stack]) stops the current block with an application-defined error that reaches the client as ORA-num plus your text — the way to signal a business-rule violation. num must be between -20000 and -20999. Pass TRUE as the third argument to add to (rather than replace) the existing error stack. Message is limited to about 2048 bytes. Callers can trap it with a named exception bound via PRAGMA EXCEPTION_INIT.

CREATE OR REPLACE PROCEDURE withdraw(p_acct NUMBER, p_amt NUMBER) AS
  v_balance NUMBER;
BEGIN
  SELECT balance INTO v_balance FROM accounts WHERE account_id = p_acct FOR UPDATE;
  IF p_amt > v_balance THEN
    RAISE_APPLICATION_ERROR(-20101,
      'Insufficient funds: balance ' || v_balance || ', requested ' || p_amt);
  END IF;
  UPDATE accounts SET balance = balance - p_amt WHERE account_id = p_acct;
END;
/