Add detailed error messages for record keys with duplicates

[yutaro-sakamoto]

This pull request add detailed error messages which are displayed when COBOL programs contain indexed record keys with duplicates.

For example, cobj displays prog.cbl:8: Error: Record keys with duplicates are not yet supported when compiling the following COBOL program because there is an indexed key TEST-KEY1 with duplicates.

       IDENTIFICATION   DIVISION.
       PROGRAM-ID.      prog.
       ENVIRONMENT      DIVISION.
       INPUT-OUTPUT     SECTION.
       FILE-CONTROL.
       SELECT TEST-FILE ASSIGN TO "./TEST-FILE"
                        ORGANIZATION IS  INDEXED
                        RECORD KEY   IS  TEST-KEY1 WITH DUPLICATES
                        ALTERNATE RECORD KEY IS ALT-KEY1
                                                ALT-KEY2.
       DATA             DIVISION.
       FILE             SECTION.
       FD TEST-FILE.
       01 TEST-REC.
          03 TEST-KEY1  PIC X(4).
          03 ALT-KEY1   PIC X(4).
          03 ALT-KEY2   PIC X(4).
       PROCEDURE        DIVISION.
           STOP RUN.