How to get errors (when occurred) when using AR_INVOICE_API_PUB.create_single_invoice

I am using AR_INVOICE_API_PUB.create_single_invoice api to create AR invoices in EBS 12.1.3 with database 11.1.0.7.0 release 11g. But if any error occurs api is not returning any error. For example if I provide invalid cust trx type id, although AR invoice is not created which is OK but any error message is not appearing. Further return status out parameter is also S (success) which is wrong. I am using following code to get error details (if occurs)

DBMS_OUTPUT.PUT_LINE('l_return_status : '||l_return_status);

IF l_return_status = 'S' THEN

dbms_output.put_line('AR Invoice Created Sucessfully : '||l_customer_trx_id);