Use the sql$get_error_text routine when you want to pass error
    text with formatted ASCII output (FAO) substitutions to your
    program for processing.

    To use the sql$get_error_text routine, you must include a buffer
    (field) in your program declarations to receive the text SQL will
    pass to it. Declare this field as a text string with a length
    sufficient to accommodate the number of characters you expect
    for the message associated with the RDB$LU_STATUS value and for
    all follow-on messages. As an option, you can declare the buffer
    length as a separate field (defined as a signed word).